Clearly I'm doing something wrong. What happens when your revive during the Mystery phase and there is a minion on the Town Hall. Fight it immediately?
Yes, fight immediately. Basically, the game never continues while a hero and minion occupy the same space (except for KO'd heroes).
The designer has posted about it here.
If so, does this happen before or after drawing the Mystery card. Not seeing where the rules address this.
It happens before you draw a mystery card. The sequence for the Mystery phase is on p. 14 of the rules.