CEZ2 is already in the game. Blue Mountain Zone doesn't even exist (map17 is just an experiment for the second act of Red Volcano that was ultimately scrapped). Rocky Mountain Zone and Mine Maze Zone have long been scrapped and replaced by Arid Canyon Zone.
Oh, Sorry for some reason I forgot Castle Egg Man Act 2 was actually in the game.
No, that wouldn't make sense at all. None of these user-made maps are anywhere near as good as the official SRB2 maps.
ACTUALLY, the maps from the specific mod I'm talking about are very well made for user-made maps. If you want I'll send you the wad file I'm talking about and you'll know what I mean.