Battery draining overnight

You have a one-wire. You should be able to run a jumper direct from the alternator output to the battery ---with a fuse/ fuse link/ breaker for protection, and IT SHOULD NOT drain the battery. If that is a one wire, and disconnecting the alternator stopped the problem, then THAT ALTERNATOR IS DEFECTIVE

And as I said earlier, the people at wherever you bought it from should have known that