Obedience can drop for many reasons: high stress(this is a common problem), low reputation in the town they work, high courage and confidence but low loyalty, failing to have enough food to feed them, having a low obedience and high courage slave instigate problems, not interacting with slaves with the Clingy trait, high lust for many days without sex, low luxury(can happen if you run out of supplies midway through the end of day), or having a dirty mansion. These are vanilla mechanics.
The housepet job and sleeping in the kennel each reduce charm a little per day. The pet rule reduces charm significantly per day. Regressive trait has no per day nor any stat effects.
Having more than one slave sleep in the master's bed is usually a recipe for a stress disaster unless you have prepared carefully. High stress can wreck slaves. The important things for sleeping together in the master's bed is a good relationship between each of the slaves (can be checked in the Statistics tab) or having the Fickle trait and not having the Monogamous trait. These are vanilla mechanics.
Having a farm manager with low confidence has a bug that will inflict stress upon everyone in your mansion.
Many jobs cause stress even if the person may be inclined to it. Fear is a partial substitute for obedience, it has no stress effect though they may gain stress while raising fear. Toxicity will occasional cause a bit of stress, but unlikely to be a problem by itself unless they are mutating.
The hybrid races are part of Ralph's Tweaks, you need to enable those before a person is created or conceived to be given a hybrid race. They can be enabled using the Constants mod and restarting after changing the setting. You may need to try a couple times anyways as with 50/50 splits the dominant race can go either way but the hybrid only happens for one race (Demon for Succubus).