It will be interesting to see how it works if it comes to that. On a vote by vote basis the stuff that the SNP does not support may well get Tory support (personally I think Labour and the Tories have more in common than Labour and the SNP, as I said above the differences are in presentation, scale and implementation, not policy except for EU).Agree the Tories have had a poor campaign to date, Stan. I think Miliband's "We're going to bash the bankers and the energy companies and mansion-dwellers" has had an understandable resonance. In practice, if he gets in, I don't think it will be anything like that easy.
The Tories would never vote down Trident or risk being accused of opportunism by threatening to do so.
I note your confidence that Labour could govern without the SNP. I can't see how, unless the Lib Dems do better than expected and throw their lot in with Labour. Support from the Greens and Plaid Cymru is miniscule. Even then, the Tories, possibly with UKIP and Northern Ireland MP's support, would ensure that Labour would limp so that, in reality, Labour will have to do deals with the SNP to get legislation through. What will the SNP demand in return? It will be self-serving for Scottish Nationalism.
Wouldn't be surprised if we see another election quite soon if the outcome is really messy, but that may not produce anything better. The Tories really have been screwed by Scotland - they (and the other 'No' parties) offered a bribe to keep it part of the union, but that didn't make the SNP go away, quite the reverse. If they had lost, Cameron would have gone, Boris would be in (one Oxford Old Etonian for another), Labour would have lost a bunch of seats.
Now we await Boris and John Major being wheeled out, before the traditional last minute threat of what will happen to the £ and the stock market if Labour get in. I don't think that works, because people resent being bullied by impersonal international markets, even though their lives are of course dominated by them anyway.