Movement question

[SsSam]

I am Pythia.
Ermor is sieging a castle of mine. The doors are open. They could likely walk into the castle against my meager defending troops.
I have an army in a province next door.
Ermor is allied with Man.
Man also has an army in a province next door.
It is my suspicion that Ermor is planing on vacating the province under siege and allowing Man to take my castle.
So....Ermor would move to a province of his own before anybody else and then Man would move on top of the castle.

It is my belief that the castle would have its defense fully restored when the Ermorian army moves away. True or False?

If I were to try and slip my army into the castle without combat what are my chances? Once the Ermorian army moves away the province is mine. Would my troops then move in the move to own province section of turn resolution or is that too simple?

Sorry for not testing on my own.