1. Patronage head girl setting raises loyalty and reduces fear. No other effects that I have seen. Haven't looked at the code though.
2. I've never had a slave get the "Fickle" trait from having sex with other slaves.
3. I usually do threesomes as there seems to be no downside and the mana harvest is the most efficient. It also leads to the slaves loving each other.
4. A loving relationship doen't seem to stop minor arguments. But they don't get serious.
5. Slaves with the 'Slutty" will sometimes try to seduce other slaves. I don't remember "Fickle" slaves doing that. It would be logical though.