I agree that I think that the alleged 'Rainbow' Alliance would have been doomed to failure, simply because that sort of multi party coalitions are fundamentally unstable. I don't entirely agree, however, that the SNP and PC would have actually held out for preserved funding for Scotland and Wales- at least on this occasion. I suspect that the bribe for them would have been precisely the same as the bribe to the Lib Dems- voting reform. That would (if Alex Salmond was sensible, which I agree is open to debate) have been majorly to their advantages in future elections- it's ludicrous that the SNP can be the largest party at Holyrood but only have 6 MPs.

Now, the Scots Labour MPs may well have objected (I'd have been surprised if they hadn't), but a well run Whips department can usually keep that sort of thing under control, with a mixture of threats and bribes!