If predictions are accurate then only the following combinations will give the 322 seats needed for a majority:
- Labour + LibDem + SNP (344)
- Labour + SNP + PC + Green (322)
- Labour + SNP + PC + Green + SDLP (325)
- Tory + Labour (547)
1-3 are out because Miliband has said Labour will not form a coalition with the SNP (he's trying to look strong by saying it, but he has no choice, it would be suicide for Labour both in England and Scotland).
4 is laughable.
The only somewhat viable option seems to be a minority Labour + LibDem coalition (293) which would just outnumber Tory + DUP + UKIP (290) and would see the SNP either vote with them or abstain (except for Trident, but that would have Tory backing anyway).
Of course if nobody can form a government we would see a second election, which would strengthen the Tories due to many voters who defected to UKIP returning after seeing UKIP fail to meet their expectations in the first election, though of course this would also see some Labour defectors return from Green/SNP, it's way too early to guess how a second election would pan out.
No because it would be political suicide for Labour to attempt that, Miliband would end his career and any hope of Labour getting re-elected for at least a decade or more, in England Labour would be seen as traitors and blacklisted by voters (we're talking "I'll never vote tory because of Thatcher" levels of hatred) and in Scotland they will have validated the SNP's "if you want Labour then vote for us and get Labour plus extra toys" campaigning which means no mentally stable Scot will ever vote Labour again.