Yeah - the MBA got him made fun of a bit because he was this fancy college guy while everybody else was a schlub. It really got drilled in the episode where Michael went to speak to Ryan's class.
As for filling a position, I would think that you would want to start interviewing before the firing to minimize downtime, but I wouldn't make it as clear as David thought it was that Jan was being replaced (unless he thought she was so slacking that she wouldn't notice. But, I don't think any interviewer would tell someone who they would be replacing - but it worked for the effect that they wanted.
As for Karen - I think that it is sort of up in the air as to how Jim resolved it, but clear that he did. He was her ride up to NYC and I assume she needs to come home, and he must have said something.
Couple of final thoughts -
1) I never thought a show relationship could get people so caught up. Lori spent all day telling me how excited she was for 8pm. When Jim walked in and asked Pam out, Lori squealed like a high school senior who just got asked to the prom. Wouldn't have thought much of it, but she managed to scare the baby.
2) Next season seems to be building on dealing with the aftershocks of failure. Karen's failed relationship with Jim, Roy's relationship wis Pam, Ryan's relationship with Michael, Ryan's relationship with Kelly, Angela with Kelly's ouster, Dwight with his planned takeover, Andy with his rise from the ashes (I mean he was a Cornell grad in anger management just a few months ago - did he think this would last?), Jan with her sanity, Michael's relationship with Jan and her...changes, etc.