"But if the tories proposed something so heinous they would prompt a no-confidence vote, would they not also risk losing the consequent election?"
I've set my Ouija Board to 1983. Voices are coming through. They're saying "we've been talking to Ed, and he thinks we have a point".
More seriously, Labour don't have anything even remotely approximating a manifesto. There are plenty of things that will convince the Lib Dems that they'll lose their seats, without being so toxic that they'll cost the Tories much in the way of seats. Labour lost the last election on economic competence (they'll look worse), not being believed (they'll look as bad) and immigration (they'll tear themselves to pieces). In an immediate election, people will vote for the devil they know will deliver stable government, even if it's horrible. ,