Also, the LD path back to relevance has two steps:
1. Win enough seats to hold the balance of power in a hung Parliament;
2. Play their cards well enough to get credit for keeping their coalition partner in check.
If they're only seeking to win about 20 seats, the window in which 1 is even possible as a strategy is incredibly narrow, especially since in those circumstances you can always promise to built a new relief road in County Antrim instead to win over the DUP.
And to demand PR at all costs to enter coalition, which would probably be easy enough to get Labour to do.
I take it you're too young to remember how things went down in 2011?
This is partly what I’m referring to. Clegg should have demanded at the very least actual PR, not settled for AV (which would have produced a less proportional result than FPTP at the election), and really the next time the party is in coalition, they ought to make it a condition of the coalition to adopt PR without a referendum.
I think if they want to take that negotiating strategy, they should probably make sure they have enough money left for a second election that year. There's a substantial chunk of Labour that opposes PR on general principles, another chunk that reacts poorly to the Lib Dems making ultimatums, and a third chunk whose openness to PR drops rapidly the closer they get to forming a government.