The base plate came off easily. Cleaned the corrosion off the base plate with some stop bath on a cotton swab. (Acidic stop bath neutralizes alkaline battery electrolyte.) Carefully swabbed out the battery compartment. No apparent damage inside. The camera was stored upright so I think the battery chemicals drained downward, sparing the inner workings of he camera.

"Liquid Wrench" did not work on the stuck battery cap. (After the base plate was removed from the camera.) Now soaking in scalding hot water. Hoping that thermal expansion will do the trick.

I noticed that there are two drilled out indentations on the inside of the battery cap. They might offer a more substantial grip on the part if I can find an appropriate tool to fit between these lugs. A wide, flat bladed screwdriver might work but I don't think I have a screwdriver wide enough. Will search my workshop to find out.

Okay, so work in progress... Thanks for your help so far!