Perhaps if Charles does become King and shows the same likelihood of longevity as his parents, he will have the gumption to say that he won't go on beyond 85 or 90. He could still be called King. (In the way that US ex Presidents are still Mr President.)
No he couldn't be. You don't have two kings.
If the Queen were to abdicate, she would become HRH the Duchess of Edinburgh
If the King did, he would become HRH Duke of SomwhereOrOther (picking whichever of the dukedoms merged with the crown that he fancied, or even creating a new one for the purpose)
It's a reason why retirement is by Regency, not by abdication. So yes he could retire. Just as the Queen can, and remain Queen by installing Charkes as Prince Regent.
We've been the new Elizabethsns without doublet and hide, but perhaps if there were a new Regency, we could have heaving bosoms in A-line muslin dresses, and men in wet shirts?