q1 - Your dominion already gives you some information. The scout is there to extend beyond your dominion's reach and see what's going on in other dominions.
q2 - Well, double-clicking on a unit type highlights all units of that type. That can help.
q3 - Undead nations get spells that bolster resistance to banishment effects. There are other nasty tricks out there as well that help undead overcome banishment. There're also some spells that Death magic gets that can target enemy commanders. That makes those priests think twice about showing up to a fight. Finally, undead multiply much faster than living units. You can only banish so many before they overrun you.
