Kirk Arrington playing drums for them is definitely out. He "retired" due to health issues related to diabetes. No offense at all to Plate, but Arrington was an absolute MONSTER of a drummer. MC without Arrington is a bit like Slayer without Lombardo. You simply can't replace him.
Jeff is actually an exceptional drummer in his own right mind you, but Kirk really put a lot of personality in his playing of the Howe and first Wayne era. As talented as Jeff is, "playing" the Arrington stuff will be no problem, but writing new material is different. He is his own man with his own style and even with Howe back, I don't know that the same distinct classic Metal Church "sound" that is quite drum oriented will happen. It is possible mind you, though. When Stephan Kaufmann left Accept, I felt like they were doomed, but Schwartzman absolutely filled the mold and was able to maintain the "Accept sound." All we can do is wait and see.....errr hear.
As far as the other musicians, Duke, Craig and John were not elite musicians by any stretch, BUT they certainly did have their own style. The Howe era Metal Church really resembled no one else. They blended their own unique style of American powermetal with copius thrash elements, that I really haven't heard much anywhere else other than some Armored Saint, some Angel Dust and Breed/Olsson era Tad Morose.
Bryant