Opinions differ on how possible/probable the options of a consultative referendum or early Holyrood election are in the event that Theresa May remains intransigent. Brian Taylor of the BBC, for example, acknowledges that both options are on the table, but insists that both are "unlikely" because Nicola Sturgeon would regard them as "gestures". Whether he's being led by his own assumptions and preconceptions, or whether he's been reliably briefed to that effect, is anyone's guess.
However, as there seems to be some confusion over exactly how an early Holyrood election can be brought about, it might be worth refreshing our memories by looking at the relevant part of the Scotland Act.
"The Presiding Officer shall propose a day for the holding of a poll if—
(a) the Parliament resolves that it should be dissolved and, if the resolution is passed on a division, the number of members voting in favour of it is not less than two-thirds of the total number of seats for members of the Parliament, or
(b) any period during which the Parliament is required under section 46 to nominate one of its members for appointment as First Minister ends without such a nomination being made."
The crucial word is "or". Options a) and b) are either/or - they don't both have to be met. In other words, if the First Minister resigns and is not replaced within 28 days, an election is triggered without the need for a two-thirds majority, and the unionist parties would not have the opportunity to form a blocking minority.
There is, however, a small catch. If there is an election for First Minister during the 28 days and only one unionist candidate is nominated, there will be an "affirmative vote" and that person will be rejected. If, however, at least two candidates come forward (say, Ruth Davidson and Kezia Dugdale), it wouldn't be possible to stop one of them from being elected. That's basically because the rules are a bit silly. If Davidson received the votes of the 31 Tory MSPs, and Dugdale received the votes of the 24 Labour MSPs, the vote would be declared valid and Davidson would technically become First Minister. That's not a problem in itself, because the "Davidson government" would, within a few short days, be ousted by a vote of no confidence. However, that would simply start the 28-day process all over again, and in theory we could go round in circles into infinity.
In practice, that wouldn't happen, because the unionist parties would be worried about making themselves look ridiculous, and people would be chanting "politics is not a game" at them. However, we should probably be prepared for them to attempt the stunt at least once. Personally, I don't think that would be the end of the world.