As the coalition debate rages, in my mind, the whole premise feasibility rests with Quebec. If you look at the various scenarios, using the facts at hand, even if the Conservatives lose an election, it is unlikely that you will have a majority of Liberal and NDP MP's.
Can you still form a coalition? Sure you can, but you still need other party support and that renders the union unstable by its nature. A much, much more desirable arrangement involves an outright majority, and the only likely scenario is a decided setback for the Bloc. Canadians will never accept an arrangement with separatists, dream as you might, it's a fundamental NON STARTER in English Canada. It is also true, that the coalition concept will be an election issue, and to think the Liberals and NDP can simply say "let's leave this discussion for after the election" requires a certain naivety. If this coalition debate is on the table, a centerpiece of the election, it is shrewd to look for ways to make it an advantage, rather than constantly defending. It is here that the Bloc and the battle for Quebec could come into play.
Quebecers are relatively amenable to the coalition concept. This openness can be exploited by the Liberals and NDP on a couple fronts. The most glaring, both parties directly target the Bloc as an obstacle to getting rid of Harper. Given that the Conservatives are least popular in Quebec, the "change" sentiment most obvious, a two pronged attack might find an audience. This strategy could be solidified by the Liberals and NDP agreeing to a non-aggression pact in Quebec. The Bloc benefits from a divided "opposition" so to speak, which result in disporportionate representation, a united alternative could well prove effective. If the Liberals and NDP made an arrangement in the province, a side effect would be to further neutralize the Conservatives in Quebec, which could compound the benefit to the "federalist" opposition. A serious discussion about a coalition isn't a negative in Quebec, in fact it could well shakeup the status quo.
As long as the Bloc wins 50 odd seats, any NDP/Liberal coalition probably won't command a majority. Understanding this real obstacle, as well as the probable Conservative attack lines, means embracing the Bloc "problem" and turning the argument around, leveraging the Bloc to advantage. Rather than fear the coalition discussion, and try to skirt around it with increasing lame talking points, the forward thinking strategy looks for ways to embrace it, and in so doing possibly increase representation.