If you have the money to reset, I would recommend boosting aspersion to 15 while leveling. It is a significant boost to damage and you don’t really need the priest survival skills till 280+. A great leveling build is:
15 asp, 15 bless, 5 mass heal, 5 stone skin, 5 exorcise.
You can put 1 point in revive to help vs. one-shots, and then skill reset when you are around 280 to put more points in revive and mass heal. Mass Heal is the only direct healing skill in the game, and while it takes some time to learn how to use, you want it as a priest 3. It has been invaluable in 280+ content with the kinds of damage being dished out.
I would argue that cleric 2 isn’t enough for healing in 280+ content, it is that significant for a priest3. Going priest3 means you are sacrificing ranks in other classes that offer mitigation like diev3. You need the extra heals to make up for that.
Last Rites is just Sacrament applied again, simple as that. If you have sacrament 1 or 0, last rites will apply another line of damage equal to sacrament 1. It is useful for leveling.
Again, you want mass heal. Get it, learn it, and love it.