1. Mewtwo was left in the game by mistake. We were supposed to remove him due to a sprite permissions problem, but forgot about the random selection.
2. Hm... I haven't seen this happen, and I just did Krystal vs. Yoshi yesterday...
3.
http://www.mediafire.com/download.php?938i33zvi3svyqp
Try this. Take the exe file and replace the one in your current game. It fixes said problem. Does your computer have a dedicated graphics card? If not, that's why you're getting the error.
4. Yeah, as far as I know, threaded loading in GM is impossible without extensions.
Hm... Idk about Jiggs. Her fall speed ties with Luigi's for lowest in the game, and she has terrific horizontal air speed, allowing her to cover a lot of distance in the air. However, I think her jumps are slightly higher than before, and perhaps she's not quite as floaty as in Brawl. If you want to wall-of-pain, it's doable. You'll have to throw in some jumps in between Fairs.