The new federal courthouse in downtown Salt Lake City is a massive, futuristic cube of metal and glass that looks imposing, austere and, above all, impregnable. Armed guards patrol the exterior 24 hours a day.

But if a certain group of polygamous religious extremists in a lonely corner of southern Utah are to be believed, this Wednesday the walls will split open and fall when one of their leaders, Lyle Jeffs, appears before the judge in a major fraud case, according to former followers of his sect.

Simultaneously, an earthquake will apparently cause the walls of a prison in Texas to crumble and Lyle’s brother, Warren Jeffs, the group’s “prophet” and supreme leader, will also walk free – despite the fact he has been serving a sentence of life plus 20 years in that state since 2011, convicted of having sex with underage girls as young as 12 that he took as polygamous wives.

By divine coincidence, perhaps, Wednesday is 6 April, the date most Mormons – and the outlawed, rejected offshoot sect of that religion known as the Fundamentalist Latter-day Saints (FLDS) – proclaim is the actual birthday of Jesus Christ.

“I am hearing from people inside the FLDS that on April 6 there is going to be a kind of apocalypse,” said Elissa Wall, who escaped from the repressive FLDS community after being forced by Warren Jeffs to marry her cousin when she was just 14. “It is prophesied.”

Thus far, however, the only signs of apocalypse have been the series of criminal and civil cases that have hit the sect all at once, leading seasoned observers of this group of fundamentalists to ask if this is the beginning of the end of the FLDS.

Lyle Jeffs, foreground, and his brother Nephi leave the federal courthouse in Salt Lake City in 2015. Lyle Jeffs is now in custody and due at a detention hearing on fraud charges on Wednesday. Photograph: Rick Bowmer/AP

Lyle Jeffs is due in court on Wednesday at a detention hearing. He is asking to be set free as he awaits trial but is considered a flight risk.

He was among almost a dozen senior figures of the FLDS who were arrested in late February in a joint raid by FBI agents and local law enforcement.

Although the FLDS has been most notorious over the decades for polygamy and child abuse, Lyle and his cohorts have been charged in a welfare fraud case, accused of swindling the federal government out of millions of dollars in federal food stamps. All the defendants deny the charges but if they lose at trial, currently scheduled for May, they face hefty prison sentences and fines.

The criminal case follows hot on the heels of a civil trial won by the Department of Justice earlier this year after it sued the remote twin towns of Hildale and Colorado City, where the FLDS is based on the Utah-Arizona border.

The towns were found to be controlled by the church, which discriminates against residents who are not part of the religion. The judge has yet to announce what sanctions will be imposed but could hand over municipal control and law enforcement duties to the county authorities, and impose heavy fines, according to Utah private investigator Sam Brower, who has assisted on several of the recent federal investigations.

And if Lyle Jeffs and his cohorts lose this fraud case they could be imprisoned and also heavily fined.

Having Lyle Jeffs in custody is already a big blow to the FLDS faithful in the twin towns because he communicates all the orders to the community issued from behind bars by Warren Jeffs, whose followers believe he directly channels the voice of God.

“I think it’s certainly the beginning of the end of the FLDS as we know it,” said Wall.

“The feds are not going to shut down the FLDS, but it’s a mindset and more and more people are leaving the religion and those that are still inside the faith are bewildered and don’t know what to do or what is going to happen next.”

To add to the pressure on the church, in the last 10 days Wall has won a crucial ruling in a civil case she has been fighting for a decade.

The supreme court of Utah cleared the way for her to sue the financial arm of the FLDS for the way the religion that she was born into treated her when she was forced into underage marriage.

If the case goes all the way it could cost the church trust up to $40m, which it estimates is a third of its assets.

Elissa Wall at the Bad Ass coffee shop in Salt Lake City. Photograph: Joanna Walters

Sitting in a coffee shop in Salt Lake City, Wall told the Guardian that if she does end up winning any money from the FLDS she plans to put substantial funds towards boosting the economy of the community she fled and helping to create jobs for those who have left the religion but want to carry on living in the far-flung community.

“I have offered to settle with the church for a small amount so many times and they’ve always refused. I’ve been fighting this case for my entire twenties,” said Wall, 29.

She was a rare witness who was prepared to testify against the church, in the first criminal case Warren Jeffs was arrested for, when he was charged as an accomplice in her underage rape and stood trial in St George in southern Utah in 2007.

He was convicted but the verdict was later overturned on a technicality.In a complicated legal history, Wall ultimately allowed further charges in her case to be dropped, so that Jeffs, who became a fugitive on the FBI’s Most Wanted list, could be extradited to Texas in the case that finally put him behind bars for life.

Now the latest civil ruling in her favor adds to the pressure on the FLDS coming from many sides, and should help to force more change in the outlaw sect, she said.

She is now an advocate and volunteer who helps people wanting to leave the restrictive sect.

Those inside the FLDS community are banned by church leaders from using the internet or having access to outside media, even newspapers, magazines, music or television.

“When people leave they are very vulnerable,” she said.

Residents walk along a street after a flash flood in Hildale, Utah, in September 2015. Photograph: David Becker/Reuters

She assists a local organization called Holding Out Help, which smuggles cellphones into the community when it receives word, usually via friends or relatives on the outside, that someone wants to escape the FLDS. The organization then helps get individuals, or sometimes whole families, to safe houses where church leaders will not be able to track them down and coerce them into returning to the religion. It also helps them find their way into education or jobs.

Tonia Tewell, the director of Holding Out Help, estimated that she has helped a further 150 people leave the FLDS in the last year and that things are changing rapidly in the twin towns, now that many people are leaving the religion but choosing to stay in the area.

They go to mainstream public schools, instead of being home-schooled mainly in religion as FLDS members are. And they shop at ordinary stores, in St George, or the few outside stores, such as a Subway sandwich shop and Dollar General store that have opened in town recently. FLDS members, by contrast, get most of their food supplies from church-controlled storehouses, which are at the center of the alleged church-operated federal food stamp scam.

There are no official figures, but FLDS members in the twin towns are estimated to be below 10,000 and getting fewer by the year as many leave.

“People are getting a lot more help these days from family members who have already left. And I get word from inside that someone wants to leave, especially young people, and I drop phones off in the bushes around town and people contact me and I help them,” said Tewell.

Many FLDS members are “at the end of their tether”, after years of the church leaders imposing strict doctrine by splitting families up and sending members outside the community, sometimes for years, to repent, if leaders such as Lyle Jeffs deemed they had offended the church in some way, she said.

“The community is absolutely paralyzed with Lyle in jail,” she said.

But she sees many other positive signs of change in the community. An ex-FLDS woman in Hildale has started up a women’s group that regularly has a few dozen at meetings and attracts 100 or more women when it holds special events.

And Hildale recently held the first political caucus in many years during a presidential race, with Republican voters turning out to caucus last month and choosing Ted Cruz as their favoured GOP nominee.

“It’s changing a lot there,” said Tewell.

Last year the FLDS and leaders, including Lyle Jeffs, were fined $2m by the Department of Labor for child labor violations.

Kat Allen, 27, a cousin of Warren Jeffs, who fled the sect after her family tried to force her at 15 to marry a man she estimates was in his late forties or early fifties and already had three wives, still has family inside the FLDS who she says are completely loyal to Jeffs.

Allen, who was born Kathwren Steed, left the community and the religion to avoid the marriage but also because she had realized she was a lesbian and was having a secret relationship with a girlfriend who was also inside the sect.

Now living near Salt Lake City and married to a woman she met outside the FLDS, Dixie Allen, whose last name she took upon marriage, she said that those still inside the faith convince themselves that their leaders, such as Warren and Lyle Jeffs, are incarcerated because of conspiracies and lies by non-believing society.

Her parents still live in Hildale.

“They are 100% loyal, no, 200% loyal to Warren Jeffs,” she said.

The FLDS dominates the remote communities of Colorado City, Arizona, and Hildale, Utah. Photograph: George Frey/Getty Images

Sitting in a bar with her wife, drinking – without irony – a locally brewed ale called Polygamy Porter, Kat Allen said that adjusting to the outside world is hard because FLDS children are told “since the day we are born” that they will burn in hell for all eternity if they break with the faith.

“For years after I struggled with that even though I knew in my head that it wasn’t true,” she said.

She is still in touch with many people inside the religion and confirmed that the message is being spread that Lyle and Warren will be sprung by divine intervention on 6 April.

She also knows many members of the sect who have left recently and are living in St George trying to adapt to a more mainstream way of life.

She said children as young as 14 inside the sect are separated from their families and made to live alone, often in trailers with no hot water, with no contact with their siblings for several years, as punishment for something as trivial as admitting “impure thoughts about the opposite sex” to church elders.

“There is a climate of fear,” she said.

Allen is relieved that the federal government is taking action.

“I’m relieved to see it. It’s definitely not as much action as there should be but just the fact that action is being taken and attention is being paid after a long, long time of the authorities turning a blind eye makes me happy,” she said.

However, as Wednesday and Lyle Jeffs’ detention hearing approaches, observers on all sides will probably be holding their breath.

Sam Brower was aghast on Friday night when a judge released one of Warren and Lyle Jeffs’ other brothers, Seth Jeffs, as he awaits trial in the fraud case where he is a co-defendant with Lyle.

Jeffs is a convicted felon, after pleading guilty in 2006 to harboring, aiding and abetting a federal fugitive

“It’s unbelievable,” Brower told the Guardian. Seth Jeffs, a convicted felon, plead guilty in 2006 to harboring, aiding and abetting a fugitive: Warren Jeffs. Brower said he would not previously have imagined there was a chance in a million that Lyle Jeffs will be released on Wednesday, but now he is not so sure.

“It’s worrying,” he said.