So after yesterday's agony of trying to clean and tighten up the original fuse block, I decided I couldn't wait any longer. I bought a new, marine fuse block with a long, dedicated grounding bar, and then because it had a large, master lug, I daisy-chained another ground bar off of that, for future circuits. I sealed everything with liquid tape and then self-vulcanizing tape around that. Instead of push-on spade type connectors, I used ring terminals with screws. The original fuse block disintigrated in my hands.
I replaced the rest of the faulty switches and now I've got everything working except for the stern and masthead lights. I found the wires, but I ran out of time and daylight.
As you all have said, there isn't a lot of wiring on a 25' boat. I have managed to inspect it all (pulled some paneling) and the actual wiring is fine. It was just at the fuseblock where things went to hell. The ends corroded and the 2nd owner just tried to bypass everything and splice it all to the battery with inline fuses in (some) of the circuits.
The way the switchbank is wired, it all passes through a master battery cutoff switch. The cutoff switch will be left on, the bilge pump switch will be left on, and the pump is a Rule that has a built-in float switch so pumping will be automated. Tada!
Once I get those lights wired up and and the new bilge pump installed, all I need to do to enjoy my boat is re-finish the teak and scrub the whole boat.