Well for me, the problem is sporadic. Sometimes, I can quit it without crashing it, sometimes I can't. And yeah I can open the MotionEtc while previewing Nana. I find it very unpredictable at times: sometimes it opens smash_sound.brsar, other times, it simply crashes.
I have this same problem too. It somehow loves to crash whenever I try quitting the preview. This happened when I was previewing the Ice Climbers. I think it's some sort of bug.
Well characters have a predetermined set of slots (assuming you don't use cBliss). All of these are completely different characters, even though they look the same with different colors. So you can have both a Cloud and a Riku texture hack on Ike. The only thing exempt from this is if you use something one-slotted like a PSA hack or certain size mods.
I'm sure there could be a way to remove character portraits from battle, but I think it has to be done through coding. Simply making the battle portrait transparent will do you no good: it will just be a colored background where the battle portrait used to be.
For the .gct file, what codes you have in there? If you saved those codes to a .txt format, you can simply add the code in and just reexport the .gct file.
To answer your last question, no, you can't do that. It will end up freezing the game for many reasons: the one I can think of is very different bone structures and animations. There's a reason that the model import can be worked specifically over Yoshi, and the only thing you can do about it is exporting the model, rerigging it over Sonic's bones, and importing the model into Sonic's file. The same applies for using animations and movesets.
Silencing a certain portion the .brsar happened to me before with no error message, and I still think it's something to do with size. The file size for character voices shouldn't exceed D0000 (it starts with a D). If it does exceed that limit, the certain character noises will be silenced out. The only character that legitimately exceeds the limit is Ganondorf FYI. Making the file size smaller than D0000 fixes the silencing.
Meh, BrawlVault wouldn't be the place to have viruses. Logically speaking, you want viruses to be in readily accessible stuff such as torrents for some programs, not in this site, unless some idiot is bored enough to do it. Even then, it will get reported regardless.
Man, thanks for this topic. It's just annoying how some people's issues are just so vague that we can barely help them. I know all of this sounds like common sense already, but people need this.