I'm not concerned about plausible ways to bring Jeff back to the show. If Community cared about being plausible it'd be a completely different show.
On the contrary, Community does care about being plausible. Every episode is grounded in reality to the point where these events could "actually" happen.
I read an interview with Dan Harmon where he stated that before he finishes each episode, he sits back and says, "If I read about this incident in the local paper, could it have been real?" He was specifically referencing the Zombie Halloween episode. If "real" zombies had attacked Greendale, it would have made national or even international news. But since it's just a bunch of kids who got sick eating taco meat, it barely makes page 7.
To me, that's part of the charm. These seemingly mundane events (a paintball game, a blanket fort, a school dance, even an RV retrofitted to be a mock space shuttle) are told to the viewer in a grandiose fashion.