Going back to this:
It's clear that the Queen is queen for life. If she became incapacitated, she would remain queen but cease carrying out the duties of the position and Charles would take them up as Prince Regent. But given how old he is, if this occurred at a time when he was also incapacitated, would Prince William become Prince Regent? And if so, what would Prince Charles be known as for that period? He would, I assume, still be the heir, whilst he lived, and would still become King on the Queen's death (even if he couldn't carry out the duties of the monarch, and William continued as Regent).
Charles would still be known by his current title(s), no reason for any change. He wouldn't be Regent unless he was doing the job. As for who would be Regent if Charles was unable, that's an interesting question. Not sure if it has to follow the line of succession, or if they can appoint whoever is deemed suitable at the time.
As for Camilla, I am and always have been in her camp, and think it's a great shame that Charles didn't marry her in the first place. But I think back in the day, the old guard were still hoping to get him married off to a foreign Royal, and Camilla's blood wasn't blue enough for them.