A 12 volt draw on battery from a one wire alternator

Well that is reasonable. I'd say there is something wrong with your meter ---or blown fuse--or you had it set up wrong---or simply were not making connections

ALSO try turning the pulley as you read the resistance reading. Maybe it's an intermittent connection.

Again.........try this. .........hook the field terminals to battery power and you should see a small but visible spark as you connect / disconnect the terminals in subdued lighting. Hell, LOL.........most of the time I can HEAR that.

This MAY BE the crux of your problem. You might want to tear the brush holders apart and examine them. "Something fishy"