1) Why not use pipe dope?
2) Where does the furnace condensate go, and does it have a neutralizer? I'm liking the idea of connecting after the trap, but I don't know if that's required. Certainly connect after the neutralizer.
3) If you don't have a shutoff on the hot side of the tank, you certainly could get drainage through an open hot pipe below the heater. Or you might get lucky and it's air locked and it doesn't run out. [Not sure how the vacuum breaker affects that, perhaps it guarantees it will drain?] I think you need to be prepared to drain the tank to below the hot water outlet level.