Hey there!
1. That he doesn't work for Universe Mode is strange. That said, some older mods will work without modification, others won't. They can usually be fixed, but it depends case by case (it usually isn't that easy, especially if you don't know your way around the insides of .pac files and so forth). Likewise, using an unmodified copy of the .pac file of official wrestlers from older games may or may not work. Often it requires some work to port them over. It's a bit of a hit and miss. Advanced mod creators may have a more detailed answer.
2. You can only replace renders for existing wrestlers at this time. Nobody has figured out how to add renders for unused character slots so far.
3. Not sure about this one. I'd assume it might be possible to port these things over, but I wouldn't know how.