I'm just learning here, so please don't think I'm arguing.
If bypassing and draining the HW tank is OK so it won't freeze, if you put AF in it and drain it, wouldn't it be empty of AF and only need minor flushing with water in the Spring?
My old tank didn't drain completely either. The 1/2 gal of AF left in got flushed with 3 gal of water 3 times and it was fine.
I'm of the AF mind because 1 time I didn't flush all lines with AF there were a few drops of water left in a low spot and the line burst. Didn't realize it until Spring after water was running all over the inside of the cabinets. A bear to replace that sucker.....LOL.