How do I field enough troops to stay offensive minded and yet defend taken provinces?

Try speed summoning Carrion Centaurs instead of Lords. Carrion Lords AFAIK don't summon more, just slightly better Vine creatures. You can summon 2 1/2 Carrion Centaurs for each lord. You'll find that arrowfire as well as banishment will take a toll on your armies, so it's best to get up alot of fodder.

Another useful method of getting more creatures is using Construct Mandragora. If an Apostate summons them you get 6 Mandragora's for 10 Nature Gems. The more nature magic you have (I.E. as on a pretender) the more Mandragora's you get a pop. (If you have a pretender with 7 Nature, he'll make 10 for 10 Nature, a pretty good deal, since Carrion commanders rarely field very many troops in this regard)

Another good method for fielding troops is to use Reanimation/Spirit Mastery to try to fill out your ranks. Alternatively if you get another income of gems (With searching and others) complimenting your armies with non-undead troops that can withstand arrowfire would be a very good idea.