"can anyone suggest a method of removing the plywood in a way that won't rip out chunks of mortar & brick?"

No unfortunately, although I am surprised the mortar and brick would come apart, usually the plywood comes off in pieces or does not even all come off without a grinder.

The plywood is secured with barbed staples, not easily detected.

You should be able to easily detect the staples by using a rare earth magnet or a magnetic stud finder like the Stud Pop. (Search them out and use a Sharpie to mark.) Then cut/ grind the staple top loop. Hopefully you don't have a large area because this will be time consuming but otherwise an easy job.

So long as the staples are made of a ferrous metal, that is.

ci_lantro will be correct of only staples. Not sure why I was thinking that there was construction adhesive used.

Appreciate the thoughtful advice. Super helpful. Milworkman - your assumption is partially accurate. From the floor - rising about 2.5 feet, there were thin plywood panels glued to thicker ones - all three sides of the closet. It's been a "fun" building. The previous owner was a city health inspector - and he simply just hid the mold instead of mediating it. Thankfully just limited to first floor apartment.