Two questions - Brad´s career switch and Jerry Espenson´s partnership

First of all, Brad Chase just disappeared from C, P & S and all of a sudden was a DA...but how realistic is it that a lawyer who was made partner and had a bright future at a prestigious law firm would become a DA...?

Secondly, Jerry Espenson was made partner i season 4 (I believe) after Katie spoke warmly of him and Denny told the selection committee that if Jerry wasn´t made partner he´d leave the firm. Then, in season 5, Jerry is up for partnership again and Denny tells him to do the closing when Denny was prosecuted for carrying a concealed weapon...in exchange for which Denny would support Jerry´s partnership. It simply doesn´t make sense!