Nah, corner outfield defense is not important enough for Mullins to get starts over Stewart, considering how much better Stewart's bat is.
Bring Mullins in as a defensive upgrade in the late innings (probably Mullins to center with Hays to the corner), but Mullins's bat doesn't play as a starting corner outfielder (nor is it good enough to push Hays to the corner).
I do think they will be the rotation, as you say. Matchups, injuries, resting guys, etc..will dictate they do this anyway.
What will be interesting to watch is how long it will be those 6 guys doing the rotation. The health of Hays and Santander, as well as the stamina of Mancini will dictate a lot of that.
However, I think O's management want to see Mancini play a lot of 1B because it makes him more tradable.
Getting 6 guys regular playing time in 5 spots in not that hard. Each guy just gets a day off every 6 games. Plus injuries will cause time off for some players.