A group of protesters known as the Nauru 19 have been sentenced to jail after a highly controversial retrial for charges stemming from a 2015 protest.

The long-running prosecution of the group relates to a 2015 demonstration against Nauru government crackdowns on judicial officers and opposition MPs, during which authorities arrested and charged 19 demonstrators.

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On Thursday, 15 members of the group – which included former MPs and presidents of Nauru and someone who was a minor at the time of the offence but tried as an adult – were sentenced by judge Daniel Fatiaki for jail terms of between three and 11 months. The sentences are suspended for 12 months after half the time has been served.

Mathew Batsiua, a lead defendant and one of the MPs whose suspension sparked the protest, was sentenced to 18 months for riot, with seven months discounted because of the delay in getting to trial, his personal circumstances and “extrajudicial punishment” in the form of a government-operated blacklist against the group.

The remaining members were also given discounted sentences for the same factors, or – in the case of two defendants – their young age.

Twelve of the 15 remained in Nauru to face the second trial, arriving at court on Thursday for sentencing wearing protest T-shirts and accompanied by family and supporters. They had been held in remand since last week, when they were found guilty of various charges including riot and assault and accessing security-restricted areas, but were acquitted of charges of disturbing the legislature.

Helen Davidson (@heldavidson) The Nauru 19 are being sentenced today. Here, the youngest member of the Nauru 19, Piroy Mau, who was a minor at the time of the offences, arrives at court. All 12 remaining members of the group have been held in remand since last week. pic.twitter.com/iFQZtzwvl2

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Australian judge Geoffrey Muecke was contracted to hear the first trial of the members who pleaded not guilty, but after reviewing the case he ordered a permanent stay on the trial, finding the government had gone to such extraordinary and “shameful” lengths to see the demonstrators punished that there was no chance of a fair trial.

The Nauru government denies the findings and says it has respected the court process.

The Australian government told Guardian Australia it had “made representations to the government of Nauru on the importance of respect for the rule of law and an independent judiciary”, specifically including the Nauru 19.

The most serious charge – and one of the most contested by critics – carried a potential penalty of imprisonment with hard labour for life, “with or without solitary confinement”.

Of the other seven members of the group, three had earlier pleaded guilty and four sought asylum overseas.

The former Nauruan president Sprent Dabwido sought asylum in Australia for cancer treatment and died earlier this year.

The Nauruan director of public prosecutions had submitted to the judge that a custodial sentence was “inevitable” and called for consecutive sentences for the charges of assault and act with intent. This was rejected by Fatiaki.

The DPP’s submissions included excerpts from the judgment against the three protesters who pleaded guilty which said the protest was “an attack on Nauru’s government structure”.

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“Such an attack on parliament could easily have escalated to a total breakdown of the rule of law, which could have escalated into nationwide chaos and social and political instability.”

The DPP put forward that several counts were aggravated, claiming the acts of riot were “deliberate and planned”, acted against police, used weapons such as rocks and fire extinguishers and was “aimed to destabilise the democracy and stability of the Republic of Nauru as a nation”.

He singled out Batsiua, saying the former MP “should have controlled his own emotions and controlled the crowd” but “chose not to”.

The defendants called for a non-custodial sentence, saying the riot offences were not in the higher range of seriousness and that the court did not need to place substantial weight on protecting the community from the offenders because they had not reoffended since the protest and were “rehabilitated and transformed”.