In 2005, the Lib Dems got votes from Labour supporters who opposed the Iraq War. Maybe they got votes from Tory voters who opposed the Iraq War as well.
You are right. In 2010, they could have done to David Cameron what the DUP did to Theresa May. The DUP continued to sit on the opposition benches throughout while Nick Clegg was quite happy to sit on David Cameron's knee. Nothing about the coalition benefitted the Lib Dems at all. They were seen as too cosy with the Tories while David Cameron was quite happy to undermine them, such as his attack on any change to the voting system.
IMO whenever somebody is asked how they will vote, they will answer as if the next question is "why?" At the moment, I can't think of a possible answer to that one.