I'll keep it simple, folks.
Trident renewal is a red herring. If it's Milliband in govt with confidence and supply from the SNP then there are enough Tories and Labour willing to vote to renew Trident to make the SNP votes unnecessary.
And, we don't actually "need" anything from the UK, we're quite happy to go it alone. We raise enough taxes and are productive enough to stand on out own two feet. Heck, there are even plenty of skilled and semi skilled poor sods floating around in the Med right now I'd be delighted to give a home to. Countries need PEOPLE to prosper. The states have done pretty well with their "poor, huddled masses" in the last few hundred years.
So once and for all, if you're all so fed up with perfidious Albion, LET US GO! Keep your Tories, UKIP and champagne socialists, let us get on with running our country for ourselves. Rant over.
ps, check out "Fixed-term Parliaments Act 2011 (c. 14)". A government can only be brought down now if they lose a vote of no confidence OR a two thirds majority of members votes that there shall be a new Parliament. Remember, once this election is done and dusted the SNP and other parties in Scotland have to do it ALL OVER AGAIN in 2016 for the Scottish Parliament.