Hello, and welcome to the forum
To answer your questions...
1) I'm sure a lot of MTA fans ARE playing the race mod until deathmatch comes out.
2) You're correct, you will be able to do pretty much everything that you can do in single player (except missions), and yes, with other people from around the world.
3) The MTA team does not support illegitimate versions of the game. Where it may be possible to get MTASA to work on an illegal version of the game, I suggest you show your support to Rockstar for giving us such a fantastic game and buy it, at least that way you know it will DEFINATELY work with MTA, and that you're not breaking the law.
4) We don't know which platforms GTA4 will be released for, but it's most likely going to come out primarily on the PS2 and PS3, and then be followed up by the PC and XBOX and XBOX360 versions, like the rest of the GTA series. You can find news on the GTA games at Rockstar's website, http://www.rockstargames.com/