Might have figured it out, and boy, it's stupid if true.
Last time I drove the car, the alternator was not charging the battery, and the battery died, this is known to be true. I took the alternator home with me, and between then and now, replaced the voltage regulator, then had it tested (successfully) at the zone.
While all that was happening, that battery (a big, high-amperage marine battery) was charged fully outside of the car.
So yesterday, I arrive, install the alt and battery, and start ****posting here and tearing out hair because the alt isn't making 14+ volts or any amperage at idle. Using a 100A battery tester showed almost no voltage drop on this battery, both while it was connected and car running, or outside.
Then, we decided to throw in a known dead-ish battery, started the car using a booster, and boom, 14.8v at idle. Engine bogs significantly when load is applied to this battery while car is running, and voltage does actually drop.
It seems like the battery being as full as it was meant the alt wasn't getting enough load to actually send out what it would normally, and basically I fixed it with the voltage regulator.
If this makes sense, then so it goes. If this sounds bizarre, I'm open to hearing alternative theories.