...ape shifting meta-human. the man in the glass box with the mask is the real Jay Garrick and zoom has him trapped for future purposes (those we dont know yet). We've seen a shape shifting meta human before, and when he became Barry Allen, he mimicked everything but the speed. So I believe the earth one Jay Garrick, is nothing more then a follower of zoom, just like earth two cisco, caitlyn and ronnie. Of course, with Earth One caitlyn consistently working with him on numerous cures and their romance building up, jay falls in love and forgets his real mission on earth one, he goes and saves people from a crashing building. Zoom realizes that one of his followers isnt sticking to the mission, and after kills him, (or phases through his chest) as a punishment for his disobediance. for jays cells getting destroyed, (very far fetched, but this makes the most sense in my opinion) its probably because his body doesnt have the regular characteristics of a speedster, so his body is deteriorating from taking all these speed enhancers.