It actually should crash even at starting campaign if the merc file is wrong - it usually does for me... but yeah, this might be it.
Finding out all the little bugs is a pain. It's never as easy as it seems, and the game never gives any clue about really serious bugs... I've found that going the map through the Campaign Map Editor is helpful, it's good at finding out bugs with the map files themselves. But if there's something wrong with armies, mercs, disasters or something like that, then it's just a guesswork from then on.
I'm fighting with a CTD-ing map right now, and I have found all sorts of bugs in the files by only guessing what can be wrong.