This is actually a very interesting suggestion. To give the King the ability to take for himself any vacant estate, completely lord tax free, which cannot be resized by the lord. Though it would create some conflict between the given lord and the king (not that such is necessarily bad), it would solve the King/Knight dichotomy without forcing him to be also a Lord and a Duke. It would also require him to be kicked out of the estate if he loses/gives up the crown.
I don't see the need to have the king kicked out of his estates when he loses the crown, as long as he loses his tax immunity. And indeed, you'd need it for it to be shrinkable by the ruler himself, with the lord able to enlarge it if he wants.
What are the new estate changes going to change about capitals, again? I seem to recall a lot of "we aren't sure yet". Will capitals be able to be in townlands now? If so, it would make sense if there was a restriction stating that the ruler can only have an estate in the capital of the realm. The king's center of activities wasn't always in huge cities, as I said, but his center of activity is by definition the kingdom's capital. The possibility of having a capital in a town would be historically plausible, and it would give the ruler a few more options to move the capital to where he comes from, just like it was historically for multiple new kings (probably when dynasties changed).
Simply give a similar message to the suggested for the King to any Duke or Lord that joins the army, and when displaying its members make a clear separation between Knight members (sent by others to serve) and Lord/Duke/King members (voluntarily participating).
There already is. Instead of having the message "X was sent by his liege to serve in your army", you get "X has assigned himself to your army". It's already clear that he does it of his own will. And people know this. I don't see why we should try anything harsher.
