We only have a paper manual and I'm not at the shop. My rusty memory tells me you need to remove the screws you marked as "B" and the mounting plate comes off with it. The wave generator is bolted to the motor and won't fit thru the small hole in the mounting plate if you remove screws marked "A". Also be careful of the paper gaskets if you want to reuse them. I suggest you buy Loctite 518 if you have a Fanuc robot.
It takes a little patience to get the wave generator back into the flexspline. Make sure you have the SK3 grease for the harmonic drive.
You will have to remaster and calibrate axis J2 but you should use single-axis master for just J2.
For reference, I attached a picture of what the grease in a bad LRMate 200iC harmonic drive looked like.