Now that my map crashes running BFII.exe, but NOT when I run modtools.exe (and there are no relevant errors to show for it), I was hoping someone could tell me exactly what the differences are between the two, and how a crash would be caused in only the latter.

Modtools is more stable (uses less processor power). Kill off some of the noncrashing bugs you see in modtools, see if it works, and then if it doesn't, let us know what you changed to cause the crash.

Well, I think I might know what is causing it, but I hope it is not the case. I recently put many of my vehicle spawns in a new layer, calling that layer for three different gamemodes (conquest, uber, xl). I did this because there are so many spawns, and I didn't want to place them all again twice over. I'll bet that vehicle spawns have to be in the same layer as the gamemode itself. I'm pretty sure that's the only thing I've changed in the gamemode that is now crashing.

Edit: Never mind, getting rid of a few errors did the trick. Sorry to waste your time.