From someone who has never played Pathfinder, but a fair share of D&d 3.5 :
First things first, I'd drop the 12 STR. 10 should largely be enough for a troller, I might even go all the way down to 8 (especially with your specific curse, thanks to which your load won't matter much). These points will be much better spent in INT (you never have too much skill points) and/or WIS (to negate the sens motive malus the 8 brings. A negotiator needs as much Sense Motive as he can get).
At lower levels, it might cause balance issues
The Monster Manual, under the general text for the Spellcasting trait (p. 10), says:
You can change the spells that a monster knows or has prepared, replacing any spell on its spell list with a spell of the same level and from the same class list. If you do so, you might cause the monster to be a greater or lesser threat than suggested by its challenge rating.
Taking your example of sacred flame vs toll the dead, the former forces a DEX save, and the latter a WIS save. Both are major saving throws, so this shouldn't be a problem.
However, using the latter vs. the former against an injured foe increases the damage output from d8s to d12s, which when used against lower level parties might increase the difficulty of the creature, as suggested in the quote above.
My gut feeling is that against Tier 1 parties, it might be better just to swap the damage type of sacred flame to necrotic for that "villain" feel, but against higher level parties, swapping it out for toll the dead probably won't make much impact.
As for the fact that you're changing the damage type of sacred flame, this won't really make much difference; if anything, it makes the spell weaker, since, as can be seen on the table of damage types from this answer, necrotic is more often resisted than radiant. If you were proposing making toll the dead deal radiant damage, that might be more cause for concern, but necrotic sacred flame should be fine.
Finally, for completeness, although I haven't seen/played through all of the official modules, I can at least say that I can't think of any characters for whom this change would make a significant difference. I think with this one, it's a case of common sense at the time on a per-NPC basis.
Best Answer
The Same as Any Living Creature
A dhampir would be healed by Sap Life because they are a living creature (despite their closeness to undeath), and because Sap Life is not a positive healing effect.
Here's from the ancestry description:
And from the negative healing ability dhampir's have:
Sap Life doesn't have the positive trait that many other healing abilities like breath of life and lay on hands do, so a dhampir's negative healing wouldn't prevent it from working and they would be healed just as any other living creature would.
A 1st-level harm with a roll of 5 would heal the dhampir for 6 points as you suggested, as long as any living creature also took damage from the spell.