Happy to help.
Actions cannot be split like that. All the Harm of an action must go to a single target, and movement cannot be interrupted by an action and then continued.
That's a bit of an edge case, and open to your interpretation. If all Harm is dealt at the same time, than you brought the necromancer to 0 at the same time as they hit you. Since the healing doesn't happen until after the harm is dealt, the necromancer is already at 0, and would be considered dead.
You do not add the dice together. A Blade rolling 2d6 has the opportunity to hit zero times, one time, or two times depending on the outcome of each individual die. So if they rolled a 3 and a 5, that's one hit (the 5). They would get to roll that one again to see if their combo continues, but not the 3 since it didn't hit.