I think that Joan had given notice back before the infamous dinner party, and she just felt that she couldn't give anyone the satisfaction of taking it back, even if she COULD take it back, given Lane's cost-cutting. (Of course, the irony in that is that Hooker almost surely makes more than she did, being male.) I hope that they find some way of keeping her involved with the show, but I'm not counting on it.
Every single American in that office except Don was thrilled that Guy met with a career-killing accident -- though other than Roger, no one was crass enough to say so out loud. I think that while Don was relieved not to lose his authority, he really felt sorry for Guy, because he appreciates talent, and didn't think that that talent resided in the man's foot. (He also probably thought, "that PDB, he trusted these snakes.") Also, the line about golf was a zinger -- I don't think that Don plays golf.
My favorite scene was the dead Cobra in the basket. The senior people at PP&L are REALLY evil, you know!