I'll leave the questions for @Game to provide more details on as I'm not 100% certain on all of them.

To address your concerns, all you need is a few points in corpse explosion. It's an insanely powerful skill that works very well with physical pierce. You don't even need to max it, and it can completely demolish even inferno cows incredibly quickly.

Summon necromancers absolutely do NOT need help clearing packs, it's one of their greatest strengths. If you don't want to use corpse explosion, then i just don't understand why not. You don't need to max it for it to be effective.