CM, for the SNP, a republic was a core principle 15 years go. politics changes.
Faslane is as much a problem for the SNP as it is for Westminster - the senior people in the SNP are not stupid, they understand that the Trident boats will need somewhere to go on day 2 of an independant Scotland, and equally they've no wish to have '5,000 jobs lost at Faslane' as the newspaper headlines on day 3 of independence.
they have a 'public' figure in mind for the timetable for the move of the boats from Faslane to Devonport - 5 years or so - and a 'private' figure of 10 years or so. the political gossip however is that the '5,000 stable, well paid jobs in the west central belt' are somewhat uppermost in the minds of the SNP for political reasons, and that if unforseen technical problems mean that the Devonport move slips further to the right, they won't be that unhappy.
they know that there are other options for the Trident boats if they push too hard, and that if the rump UK were to take up those options - Kings Bay and Devonport - HMNB Clyde would close in a year, and the SNP would be in very, very deep political poo because of it.
they are politicians, and politicians want to get relected - tanking the economy of half of Glasgow is going to make that difficult, so they unlikely to do it if they can find a fudge...