I think Cumberlands is pretty happy in the NAIA along with their rivals in the Mid-South conference. The Carnegie data is a little misleading. They have only 1410 undergrads on campus:
https://www.ucumberlands.edu/about/quick-facts

As for your other question, why would they? D2 has rules about playing non-D2s, and if they allowed selected western D2s out of the rules then everyone else would want to be as well (to get easier schedules?). I think SOU and EOU are pretty happy in the Frontier. D2 has an advantage in scholarships, that translates to tougher games and more losses for SOU and EOU, and that would joepardize their post season hopes. So why on earth would they want want you're proposing?