1. Temples spread dominion: each temple generates one "temple check." So yes, there definitely is a benefit to building temples, even with a pretender with 10 dominion. Increasing max. dominion is just one thing temples do.
2. Build temples.
Also, preach in those provinces.
3. Blood sacrifices generate one temple check for each blood slave sacrificed. Their effect is not limited to the province they are in. Consider them as building a one-turn, temporary temple in that province for dominion spread purposes, for each blood slave sacrificed.