[Alneyan]

On the topic of sieging: you can destroy your lab when besieged (and can only do so when you are besieged), but you cannot destroy your other buildings, no matter what the circumstances are. Of course, any enemy will have no trouble destroying your temples. The magic sites are indeed used by the besieged, and your formula for supplies makes more sense Taqwus.

You do not need to have an adjacent province so long as you do not rout, so you will be fine in the first case (unless you rout of course). Magical movement, including teleporting spells and remote summons, happen before mundane movement, so your birds would seize that province before the battle of your army. If that army happens to lose, they would be able to retreat to the newly conquered province.