Glad to see that Harmon returning remains a possibility. I agree with the "I'll believe it when I see it" crowd. If he did return I think it would give a lot of credibility for the show back to the hardcore fans who were underwhelmed by season 4.
As for issues with Jeff being around Greendale, not worried about that at all. Given how few of the episodes outside of season 1 have actually involved a specific class or professor.