The debate over the pace and intensity of an independent Scotland's transition to statehood and EU membership has taken another twist and turn, this time in the Scottish government's favour: one of the UK government's legal adviser agrees that it could only take 18 months.

On Monday morning, as the UK government prepared to publish its legal advice on the UK and Scottish independence, one of its advisers, Professor James Crawford was doing the rounds of BBC radio studios after finding himself under oblique, indirect attack from Nicola Sturgeon for apparent "breathtaking arrogance".

She had just seen the UK government capturing headlines which disclosed Crawford and his colleague Professor Alan Boyle believed an independent Scotland would both inherit none of the UK's current treaty rights and membership, and secondly, would therefore face a complicated few years rejoining the EU, Nato, the IMF and UN - on new terms in every case.

Crawford, a New Zealander, and an eminent specialist in international law and the formation of new states at Cambridge university, was asked on the Today programme whether the Scottish government was right to suggest (last week) that full independence might be achieved within 18 months of a 'yes' vote.

Well yes, he thought that was fine even though, to Alex Salmond's difficulty, that did not mean Scotland had any prior rights to the UK's current EU membership rights, particularly on the £3bn rebate. Nor would it have the UK's rights in any other international body.

Crawford said:

Treaties might be continued by agreement but then the principle is right: Scotland becomes a new states and will have to apply to membership of the UN – of course it will be admitted; that's not a big issue, but it's not automatic. Similarly in relation to the EU, there are things that have to be worked out – number of members, whether Scotland can enjoy the British opt out, things like that, so there's a process which assumes that the rest of the UK remains and Scotland is a new state.

And how long would all that take – because there must be thousands of treaties, John Humphries asked. Crawford continued:

The thousands of treaties aren't going to be a major issue but the membership of international organisations is, and it's something which is going to have to be done on a case by case basis.

And it will take how long, Humphries asked?

The Scottish estimate is about 18 months and that seems realistic.

Last week, the chairman of the Better Together campaign, Alistair Darling, and the SNP's opponents had been scathing about the "totally inaccurate" suggestion by Nicola Sturgeon, the deputy first minister, that Scotland could declare "independence day" in March 2016. (Previous leaks suggest mid-October 2014 is the preferred time for the referendum: Alex Salmond is due to confirm the precise date in March.)

Sturgeon was basing that estimate on 30 post-war independence cases, where the average time to independence was 15 months, but those included countries such as Chad, Mali, Samoa and Macedonia.

That invited ridicule from her opponents. The Telegraph reported that Darling said their list only included two EU member states, Slovenia and Estonia, which had taken eight and nine years to negotiate EU membership.

Crawford said Scotland already complied with the main tests of EU qualification – the aquis – because, as part of a long-standing EU member, the UK, all its current laws and institutions were EU complaint.

As such, it would be relatively straightforward. But it was not necessarily seamless nor, as Salmond has previously said, was it automatic: every other one of the 27 EU's members would need to agree:

The problem is that it's not just the UK because the question of Europe is a question to be negotiated with other European states.

So on that crucial matter of the timescales facing Scotland's accession, he undermined Darling's attacks and took the pace out of the UK government's spin about his analysis. Even so, other lawyers and political analysts believe it is that issue – the agreement of every other 27 member state – which is Scotland's far more trickier problem.

After the repeated statements from Jose Manuel Barroso, the European Commission's president, that part of an existing member which split off, would be treated as a wholly new state and wholly new applicant, this is a crucial political and legal issue.

Barroso's statements have been taken by anti-independence campaigners to imply that Scotland could only seek membership after independence, and not before. Senior and influential EU experts, including the Scottish former European Court of Justice judge Sir David Edward, believe Barroso is wrong. And, as Lord Wallace, the Advocate General admitted today, they have told the UK government this.

They insist the EU is pragmatic and flexible enough – particularly since there are no clear rules on such cases - to devise a mechanism which protects Scotland's de facto EU membership while it thrashes out its precise terms. What isn't necessarily clear is the timescales involved, and the complications of dealing with the national, selfish interests of each 27 member state.

Interviewed next on BBC Radio Scotland's Good Morning Scotland programme, Crawford appeared too to agree that Scotland would remain within the EU while those negotiations took place.

The question was ambiguous, failing to pin him down definitively on whether Barroso was right that Scotland needed to start from scratch after "independence day", but asked whether Salmond and Sturgeon were right to say those talks started off as Scotland being within the union, he said:

That's certainly true: Scotland is part of the UK continues within the EU but Scotland as a new state will have to become a member of the EU by a treaty of accession and therefore there are things to negotiate. As I say, it's not to suggest that this process is necessarily going to be very difficult, because Scotland already complies with the aquis now as part of the UK, but still that process has to be gone through.

But then that all raises one tricky question for Sturgeon, about her attacks on Sunday on Crawford and Boyle's analysis, as "breathtaking arrogance" and "a near colonial attitude".

While urging her opponents to act with dignity and respect, had she launched a preemptive attack on two senior lawyers, with international reputations, without hearing them out first or without reading their analysis? Academics at Monday's launch of the UK government paper believed she had been extremely intemperate.

Pressed on the Today programme about this language, she insisted that was directed at the UK government's attempt to spin Crawford and Boyle's analysis; they were exaggerating and misusing their legal opinion, she said.

Well, I don't question the motivation or the credentials of James and his colleague but I do think this is an incredibly arrogant attitude for the UK government to take: that somehow they keep all the rights of the UK and Scotland gets nothing. It rather shatters any suggestion that Scotland is an equal partner in the UK. I don't question James's credentials because there are other experts who say the exact opposite: Professor David Sheffer for example, an international legal expert, former US ambassador who takes the opposite view which says Scotland and the rest of the UK would be equal successor states.

That was exactly what she appeared to be doing: questioning their credentials. Her original statement on Sunday (which the Scottish government has not posted on its website) directly cited Crawford and Boyle's opinion.

Crawford was asked directly about this on Good Morning Scotland. After all, the UK government's position was based on Crawford and Boyle's opinion: it was they who insist that the weight of recent precedents shows only the UK is a successor state despite all Salmond's and Sturgeon's assertions to the contrary.

So, with some dignity, he carefully replied: