1) I dont play online ranked but I think you can only use the official 2k roster with injuries. in online leagues you can use other roster.
2) Still think quick match is the most used. Online leagues are kinda broken, guess way more people would play them otherwise.
MyPark and Jordan Rec Center with your MyPlayer - always packed in there.
3) not that I know.
4) dont play it but there is a mode called "road to the playoffs" that should be similar to seasons. you have to win a certain amount of online games aganst other user to get to the next seed or stay at your current.
I think some of the challenges are also online against real user.
sorry, not exactly my game modes, but hope I could at least offer some help