Well, she didn't go down without one last fight for the season.
I ran the boat one last time for the season. Put it into gear as well. Seems smoother than before so that's a plus. There was some humming and whining in reverse but forward was very smooth, so I suspect that's just the nature of reverse on an inboard.
The next morning, the bilge was full of transmission fluid. I believe I simply overfilled it. I put in about two quarts, maybe more. I think it only needs about 1. It continued to leak a bit into the next day, after I sucked some of the fluid out as well. I figure it'll weep out until it's at the right level.
Finally, I winterized and covered the boat, and that's where it'll stay for now. The transmission had leaked a little more the night before, but it seems to be closer to the normal level now. As long as it doesn't end up low on fluid, which it shouldn't, I'm happy.
The Hurth HSW 450 D, from what I can find, should hold 2.12 quarts (two liters). If the fluid is clear you are probably OK but one thought was water in the bottom of the gear raised the fluid level after it phase separated. Water source could be the transmission fluid cooler. (Not sure if your cooler is on the suction or discharge side of the raw water pump.) Here is the link where I found the capacity: https://www.simplicity-marine.com/ZF/ZF45C/Technical.htm
Not trying to be an alarmist but water, if present, doesn't need to be left in the gear over the winter.
Note, a ZF 45C is the same as a HSW 450 D per the top of the page (and the link).
+1 on what crhodes said, I wonder if there is a way to put a small amount of air pressure (1-2psi) maybe thru the vent and see if it will show where the leak is. A small leak could turn into a big leak out on the water and leave you looking for a tow and do damage to the gear.
There is an additive you can get at auto parts stores, put it in and check with the black light that comes with the kit, it will show up leaks. The additive is specific to oil vs water type leaks, so get the right one.