TL;DR - the king owns all lands. Any lords appointed are only custodians on behalf of the king, just as you are the custodian of Winterbourne Stoke on behalf of the Count.
All of Logres belongs solely to the king. Roderick manages Salisbury on the King's behalf through a grant that was given to one of his ancestors. You, in turn, manage Winterbourne Stoke on behalf of the Count, through a grant handed down through your ancestors. Uther holds all rights, etc. If Uther is made high king, then the other kingdoms, like Silchester, retain their rights as kings, but pay tribute to the High King and follow his leadership, within reason.
Count Roderick has the right to revoke the grant for Winterbourne Stoke, as Uther has the right to revoke the grant for Cornwall. Now, there would have to be huge reasons for doing so - failing to answer a muster might qualify - so it cannot be done at a whim. Vortigern did that, and it led to open rebellion. And a lord is unlikely to simply hand back land that has been in their family for generations without a fight.
On a related note, you cannot muster your levy. Only the king can do that, so when you call out the peasants for war, it is on behalf of the king. If you did it on your own for your own purposes, they would be right in refusing you (though you might visit consequences on them for that refusal). However, it is not like the king publishes a proclamation. Realistically the peasants do what you tell them, though there may be consequences if you muster a levy without direction to do so.
If everything had gone as it should have, you would answer the Roderick only. Now, however, Uther has left himself the option to override Roderick's orders directly. Also, if the King calls a muster and Roderick refuses, you are free (and obligated) to answer it individually. That's not a problem, exactly. But say Roderick refuses to answer a muster because Salisbury is being attacked. There, you have a conflict as you owe fealty to Roderick for your lands, but the King for your knighthood.
In the case of Cornwall and the others who did not answer the muster, there could be other things going on, they may be passively resisting Uther's kingship, or they may be looking to change loyalties. That's an intrigue Uther will need to sort out in the winter.
