NO Prime Minister can be UNELECTED. Check your constitutional law.
Furthermore IF Labour and the Lib Dems form an alliance along with the fringe parties they would have a small MAJORITY so they aren't trying to form a minority government. That is an option open to the Tories only
If Clegg gets into bed with the Tories all hell will break loose.
For all his supposed success at winning over new fans as a result of the leaders debates Clegg will alienate the lot of them if he does strike up a deal with the Conservatives.
I really don't see how a deal could be worked out...