[Number4]

It isn't an direct answer to your question, but for me any non-roster invitees are to take some injury risk off my starters. Any batter expected to start the regular season starts every 2nd day, which with 9 starters and 4 bench players leaves 5 slots to be taken by AAA players. I then throw a couple or warm bodies in, so there are always enough batters available for late game changes. Those rookies can also share the times they relief an starter.
Only exeption is when a batter is converted to a new position, then he'll play there all the time.

For pitchers, I go with a strict 6 man rotation and throw a couple of bullpen arms in, for good measure.

I think that should give me enough spring at bats, while sometimes a rookie takes the bullet instead of my valuable starters. And maybe one of those develops or impresses me in his at bats.