Geeker and Plutocrat - I think you've got 4d exactly right: reversed 4 (muscle group), 3 (commit crime), with the whole clue an &lit. Very clever ... but like so many others this week, almost impossible to get without multiple crossers, which I find annoying.
I still can't parse 12a. Again, there's only one possible answer, but I can't see how you get there from the clue. Any ideas?