the SNP are nothing if not clever - by making their demands so unpalletable they put themselves in a win-win situation:
if a UK party were to accept their conditions in return for a coalition it would provoke massive anti-Scottish feeling in the rest of the UK, making a break-up far more likely, while if the SNP were told to **** off and the UK parties formed whatever unholy coalition was required to form a UK government without them, they could return to Scotland and say 'look how they treat the people you elect - they'll do anything to avoid listening to you' making a break-up more likely...
personally i think the chances of a SNP coalition are vanishingly small - simply because to agree to their demands on austerity for England - spending for Scotland, not to even go near the issue of Trident, would be political suicide for an party that primarily relied on English constituancies for its MP's.