Update: got a noble from umbra, already branded, set her residence to communal, set head girl to patronage, improved noble's mental stats to 100 for courage, confidence and wit, cast Domination to lower obedience. Next day, increase in obedience: 9->30. next day: 30->??. Next day: ??->80ish.
It's safe to assume that the Authority trait doesn't stack as no 50+ increase in obedience happens. Unless daily obedience increase is capped. Which I'm pretty sure it isn't.