Okay, this is interesting. M4 has a super-duper hack-y SEXP to spawn explosions around the player, simulating a bigger and more exciting battle than the game's balancing (particular the capship speed) can actually visually produce. It is absolutely possible that what we did back in the day is highly illegal in newer versions of FSOpen. As I wrote in the other thread, all I can say is that it did work at some point (half a lifetime ago). I do not even remember the details of this particular function anymore, I recall only discussing how to realize this with FUBAR-BHDR.
M6 on the other hand is the assault on the Earth Alliance Base, where the Harvester is supposed to kill the engines of that carrier with its beams. I vividly remember the existance of a problematic bug in this mission but do not remember what exactly it was. Also, since I lacked the wisdom of proper versioning back in the day, there is no way to know what file you actually have. However, having an actual SEXP crash is something I do not remember, so that is that.
So, as before: Do all the changes you want, all of my TBP-work is in the community-domain. That includes fix-releases, re-releases, re-mixes, fixing the spelling errors and other stuff as long as you include the original credits.