Assuming there is a by-election in Oldham East and Saddleworth once the appeals process is complete (and I'm less sure about that than some - if telling lies about the intentions and policies of your opponents is really to invalidate elections, I'd think most of the House of Commons ought to be sweating), then the Conservatives should fight it hard and fight to win. Being only 2,500 votes behind under this scenario - in which Labour is badly damaged in the seat and the Lib Dems are suffering nationally - is certainly close enough to be in with a good chance in principle.
Furthermore, I don't believe it's true that the Coalition would be weakened if the Conservatives had an extra seat. From the Lib Dem side (the Conservatives didn't need it at all) the Coalition reflects the electoral reality that the Lib Dems couldn't achieve the numbers to form a Coalition with Labour, and what passes for "legitimacy" in the mid of a Lib Dem - namely that we got the most votes. If we get yet more votes and yet more seats, that strengthens the case for the Lib Dems being in Coalition with us.
Also, our proper aspiration ought to be to achieve an overall majority. We're close already. A few by-election wins, a few defections, and we'd be home.
And if the Lib Dems lose badly and choose to waltz off - so what? What exactly do we need them for now? Their sole useful function was to nod along as the spending cuts were announced. If they've bored of the arrangement and prefer to try to combine with Labour to force a General Election, then we've certainly stuck to our half of the bargain. The Lib Dems would suffer very badly under such a scenario (not least because the three most important of them would probably stay with the Conservatives).
So, we have a chance to win. If we win, it will strengthen the Coalition. And if it didn't and the Lib Dems decided to split their party and try to force a General Election, even better. Fighting hard is a win, win, win for Conservatives. The only people that stand to gain from us not fighting hard are Labour, the Lib Dems, and those associated with the Conservative Party that prefer us not to have an overall majority but instead be handcuffed in Coalition - and I don't see why the interests of any of these groups should be foremost in our minds.