Yes, the low level units go away early. They dont give much experience to kill, so I dont save them for spawns.
If a monster army takes a city its destroyed. Should happen more often (at least on Challenging and beyond).
The monster lairs will spawn armies, and I have heard of random events that makes new monster lairs (but they are so rare, that I have only heard about it). If you have cleared an area the general rule is that its clean. Its not like Warlock: Master of the Arcane where Ogres spawn on your doorstep (this works fine in Warlock, but would be no fun in FE).