IMO you get the the "added damage" on a necro fireball but you collect healing *once* (for one target & the necro half of the damage) regardless of how many people you nuke. There are certainly people who have other (reasonable) interpretations, but I think allowing a necro fireball where you get to reap healing from everybody is grossly out of line with other advanced spell options...much less other healing options.

Again these are only my interpretations. Mileage will no doubt vary by Judge till we get some rulings...