1) Very possible. You can create other human managers (what they are called in the game) that would be your friends and then put them as GM of the teams they want to control.
The CPU will control the rest of the teams by default.
Then, you set up what their in-game persona has control over (GM's can delegate anything in the organization to their personnel, basically giving CPU control based on that person's traits).
For example, maybe you'd allow delegating managing minors lineups but not organization promotions/demotions through the system. In fact, you could forbid direct influence over lineups, team strategy, and pitching staff (that must be delegated) to play truly GM only.
The only things they could touch would be drafting, finances, organizational moves of players and personnel, and player movement (signings, extensions, trades, waivers, etc.)
I played in an online league once that forbid changing strategy. To change strategy, you had to "hire a new manager" (this was way back before OOTP was as fleshed out as it is now).
OOTP would now organically allow such a restriction.
2) You can be GM only (that's how I play the game). Each human manager has a role setting. You can set it to GM, manager, or both.