It works as intended (so it will replace Kingdom of X by Sultanate of X as cultures change), it just doesn't have the feature of being able to override that prefix.
He inherited a title that was of a higher rank than the one he already had, so he then takes on the liege that this title had(so then in this case no liege At all/independent).
Well, it depends on your perspective
You need the necessary tech and requirements to do so.
Yes, realm bonuses on buildings will stack. One thing to note, though, is that if you build those buildings in a vassal's land, it may give him the realm bonus for those military units, but not you. That's an interesting question that probably requires confirmation from a developer.
If a vassals buildings also count for then a big empire with a lot Of duchies would get a ludicrously superpowered retinue,so I think it's safe to say that vassal buildings shouldn't count.
Given the death rate among knights, you should be grateful!
Not really. Most knighs have a prowess of 15 or less. I was over 30. Also, if you don't want to risk it, you could always say that battlefields are not your cup of tea at the cost of some approval or prestige and go back to whatever your education and focus are.
