1. Besides the one trait that gives minions 30% extra damage, does any other attribute/trait affect minion damage?
Nope. No traits and no attributes (minions don't scale with your stats).
2. Since most of the sigils/runes in this build are tanky/healer to keep your minions alive, how does the DPS of this build compare to Power/Condition necro builds in PVE? Having so many minions all apply DPS (assuming they all attack) is a decent amount of DPS, but does Power/Condition do significantly more?
Single target, the damage will be comparable to a condition build as long as all your minions are attacking. You also have the added advantage of not being useless if another party member/person is running bleeds as well. I don't have much experience with power builds, so I can't give you a definitive answer on that. From the little I experience I do have, I'd say mm does more damage than power builds (that aren't 100% glass cannon), but I have to say again that I have almost no clue when it comes to them.
AoE damage-wise, condition necros do much, much more damage, both with stronger staff skills and epidemic. Power builds hardly even have AoE damage and rely on staff if they have any at all, so as long as you use apothecary armor/stats, your marks should still outdamage the marks of a power build.