This is what I was trying to get at with the OP and I believe would solve all their issues if they went this way. They have a few months to change their minds before units/games start shipping to stores, but only 2-3 maybe tops.
I really don't mind either way, but I know that having this option would win them a lot more love with consumers for this generation. Let next generation be all digital based where discs mean nothing.
Good for gamers, but probably not what publishers want.
Basically you have a second copy of the game to be played at the same time.
1. System can't know that the game is already in use.
2. The system can do a check if it's not already registered, but you can simply play it by logging out.
3. Same as 1, game could have been shared/given away already.
Easier: Disc is the master copy and always works for offline single player, there can be 1 registered digital copy that can access online features which is tied to your account.
1. No internet, can always play the single player with the disc.
2. Have internet, disc in drive. If the game is not registered to the current account, give the option Register or Play offline
If you Register to be able to play online, whoever had the registered digital copy loses access to the game.
Play offline locks out online to the game. (Not to the whole system)
So someone else can still be playing offline single player from your disc while you're playing online, but why not. Movies give you 3 copies nowadays, blu-ray + dvd + digital copy. What's so bad about 2 people playing single player from one disc. Only 1 can play online with it.