how did the nobles treat their personal servants.

[Russ]

Would adding considerably more religious freedom break the willing suspension of disbelief?
The central religious dogma in the old world ( the equivalent of our Europe, North Africa, Levant and Arabian peninsular) is that all gods and goddesses are real and you can worship however you want even non human gods and malevolent ones. The big exception is worshiping the god of darkness, and the punishment is death. Some lords respect that, other expect the servants in there household to worship the god they personally worship. The duchess who is one of the main character is somewhere in the middle , she doesn't force her opinion on her underlings, but she is definitely more respectful toward the people who worship the same aspect of the same goddess.

I don't think so. History is full of cultures/times/places and religions that were tolerant of a wide variety of beliefs.

I think those kind of cultures are refreshing to explore and be reminded about.