For an answer to your second question: I have no idea, I didn't even realize Mitch was on the Gas Man's show regularly; i'm listening to online radio on my Ipod Touch or XM that time of day.
I can answer the first question though: IIRC Groz had some kind of non compete clause or something like that either from his KJR contract or something related to KIRO, I forget which; I think Dave's starting on 710 in August or September, although I would have to check online to make sure.