Ok. So the good news: problem fixed. Bad news: Threw parts at the problem and now have 2 spare alternators.
Turns out the problem was with the fuse itself. I disconnected the output wire and ran a different one straight to the battery (after the new alternator didn't work), which worked. Ohm'd across all connections yet again, including fuse terminals, no resistance. Took all connections off at the fuse/starter, cleaned (they looked good). Still no go. Then just took the fuse out and it worked.
Turns out the fuse was bad internally, but still making enough of a connection to pass voltage without a drop if there was no load; when the engine was on, I didn't have enough access to test voltage across the fuse terminals. Had I done that I would have seen my problem. Reason that I was getting 24v from the alt output was because it wasn't reaching the battery, like
@Bt Doctur said. I had battery voltage at the other side of the fuse because the battery was feeding it. Worst of all, the tach output doesn't look like it's going to work for my tach.
Looked good from the outside:
Pics of contact surfaces of both plates and the fuseable link connecting them. You can see pitting; the black is carbon tracking from arcing.
So here's the root cause: the insulating sleeve protruded too far from the link, so with everything tightened down there still wasn't enough pressure for the link to contact both plates securely. Wondering if it's missing a washer or something there--that would probably cure the issue.
