You should be able to pull the backing plate right off, but you would need to disconnect the cable.� Although, I doubt that's what's holding you back.�
In your second photo, Yes...that is the seal.� Don't dig too deep with your puller.� There is a ridge that is part of the TC� that the seal sits against.� If you get the puller behind that seal, you will never get the seal out.
Also, the seal is a good idea, but with that much oil build up, you want to make sure the seal surface on the companion flange is smooth as a babies butt.� If ridged or pitted, you should install a Speedy Sleeve.
And, use some Permatex 2 on the output shaft splines.�
All above should help keep the oil loss down to a minimum...and make the park brake work better.

Tim