3: Max Onslaught's sound data does not allow it to play. Does a fix exist for this?
I had max onslaught on my PAL version and it didn't play the sounds either.
This is because it makes use of the NTSC setup of the links to the sounds. In the PAL version those links are different so that's why no sounds are heard at all.
You'd have to fix the links in order to get the sounds working.
16: The custom percentage numbers I've made have a black background in-game, even though in photoshop the background is transparent. What is the cause of this, and is there a fix?
That would depend. Is the file you used a PNG? If so you might want to fix that and use .tga i believe. That should fix it.