What skills should Necromancers take when facing Orcs on a very large map?

The only one? Come on.

Nonsense. Conditionally speaking, you'll clear twice as much territory in the same amount of time and gain twice as much experience in total, while each hero will get the same amount.

THAT'S exactly nonsense, I don't see the point, and in any part of the game, I only use one hero.

Added 1 minute ago

Sector 2 of Gaza: In reality, there's a decent bonus to Town Experience (TE) there.

No, it's not a decent bonus, reread the manual; it's written in plain Russian that it only gives a few TE per Necromancer hero, even with level 4 Necromancy. If he's around level 20, then it's more or less worthwhile, but you still have to level him up, and I don't think it's worth it for just a few extra dozen TE.