Questions about the gameplay of Heroes 6.

Regarding the Hall of Heroes: the new hero has more troops than we can build in a day (the idea is to transfer troops from a secondary hero to the main one) + he can collect resources for us, i.e., save time. On the bridge map, this might even be more useful: even one day can significantly affect the outcome of the battle: will we gain experience from two champions in the center of the map or from the enemy, or will we share it equally.
A stack is a single unit, a single combat entity consisting of several units performing the same command.