The difference is simple. If an English Duke inherits a French Duchy, that Duchy is still, legally-speaking, a vassal of the French crown. As such, it makes no sense for the English King to revoke it and re-grant it to someone else, any more than it would make sense for him to just unilaterally decree that his son was now the Duke of some Duchy in Germany that was under the control of some other ruler. No-one would recognise his authority to do so and, as such, the revocation or grant would have no practical effect, aside perhaps from giving the person in question a (rather dubious) claim on the title.
Conversely, if Asturias Holy Wars the Umayyads for Portucale and wins, then they own that Duchy by right of conquest, in the same way as William the Conqueror owned England by right of conquest. As such, the Duchy is seen as theirs by everyone relevant, and they are free to dispose of it as they wish.
The way I was thinking about it, the de jure empires, kingdoms and duchies would play no role in the process. It would simply be a question of how you obtained the land. If you obtained it by conquest, then it's yours, you can do whatever you like with it. If, on the other hand, you obtained it by inheritance, then whatever conditions were previously attached to the title continue to be attached to it. I.e., if the previous holder of the title was a vassal of someone other than your current liege, then you still owe fealty for that title to that person, unless you choose to explicitly declare independence (and fight for it). If you lost, then obviously the title could be revoked.
Agreed, which is why it's an ideal situation, not one I expect to see implemented in CK2.
That is an interesting edge case, yes. In that case, I'd say that, for gameplay purposes, it would be part of England. You conquered the land by right of conquest, and the person who you were conquering it for agreed (implicitly) to pay homage to you for it.
If such a thing happened in reality, the King would almost certainly talk to the Duke beforehand and say "I'm pushing this on your behalf, provided you accept that it's now part of England", and the Duke would say "OK, sure". The whole "pushing your vassals' claims on another realm" thing is actually somewhat questionable historically (I can't really think of it happening to a united country, although it certainly did happen to an extent with the Welsh and Irish), but I think it's a gameplay abstraction that is necessary to handle issues with what people would and would not have considered a "justified" claim on land.
It depends on the relative strength of the overlord. France in the time period of the Hundred Years' War was in a position where the King had little real power. As a result, powerful Dukes could and did push to increase their own power. Of course, the overlord doesnt have to recognise it, and that is represented in-game by the ability to revoke Duchies.
The ability to sub-grant land was absolutely part of the feudal system, though. It was obviously not any more feasible for a Duke to control all of the land he was allocated than it was for a King to do so, so the Duke was expected to pass that land on to lower-level retainers. The King didn't particularly care what the Duke did, because the King already granted that Duke the land to do with as they wished, and giving away bits of it was a necessary part of that.
Yes, which is why just occupying territory in a war doesn't give you the title to it. However, when you win a war with a defined CB, the peace treaty that is signed formally grants you control of that land. No-one can dispute the legality of that, even if they may well dispute the outcome by force later.
The existence of former holders of a title is represented by the "claim" system. If the former count of Rouen wants his land back, then he can push the claim in a war (or his liege can). And, generally, the sort of claims you're talking about with the UK and Spain are purely for show. The British Kings formally claimed the French throne all the way up until the French Revolution (at which point the claim became moot), but they made no genuine effort to enforce that claim after the Hundred Years' War, and no-one would have really accepted the claim had they tried.
