Well, Ted was exactly right: After a couple of hours of trying alternatives, I finally cut an access door in the vertical face of the riser on which the toilet sits. I cut it 7/8" down from the top edge (which turned out to be exactly even with the bottom of the ply under the head) and 1 1/2" up from the bottom (so my Bosch sabre saw would be happy).
It was hard to cut in the area of the bolts (the ones that would not come out) but once done I discovered exactly what I suspected -- that the toilet was held on with four 3/16" bolts. The access door that I had cut was just big enought for my hand and a wrench and presto, they all came out relatively easily.
I must add, however, that the bolts had never been properly bedded so there was evident of water seeping everywhere -- including a bit of crumbly copper foil!!!!!! Anyone know what that was to or from? I suspect it was heading toward the ground plate which is on the opposite bilge. If so, it has been "toast" for a long time!
I am going to soak the ply in a couple of coats of CPES
epoxy to seal it well before plugging the holes and installing the LAVAC. Pictures when done.
Again, thanks to everyone for their help, Jay