I think there is another possibility. Charles and Camilla knew they could not get married from the moment they met for two reasons: First, at the time there was still a virgin requirement (in part because of stories from ex boyfriends being unseemly for a future Queen). Camilla was likely not a virgin when she met Charles.
Second, she was not considered aristocratic enough to marry a future King. Queen Mary was born a Princess. Elizabeth Bowes-Lyon was born a commoner and then part of the aristocracy at a time when future kings were supposed to marry other royals, but she did not marry someone who was expected to be King. She was acceptable for Bertie, who was never supposed to be crowned. Her merely being part of the aristocracy (and not royal) was new for a British consort. Queen Elizabeth married someone born a Prince. When it came time for Charles to marry, I believe he was expected to marry a royal or someone higher in the aristocracy than Camilla Shand.
Thus they both knew she was “unsuitable” from the start.
But they clicked. Can you imagine meeting someone, clicking, knowing you cannot be with them, and yet falling in love anyway?
She was also interested in Andrew PB and married him for several reasons, including trying to move on a build a life because she knew she and Charles would not be allowed to marry. She married Andrew, tried to move on, and set a tone that Charles should as well.
So Charles did his duty and married an higher-level aristocratic girl, since aristocrats “knew the rules” — that marriages at that level were more about marrying the “right” person from a public perspective. There would be a public marriage and private pleasure, as that had been common in those circles for a very long time.
When Charles and Camilla reconnected, they were still operating under those unspoken rules. Since they could not have married each other, Charles had the public marriage as expected, and saw Camilla for private pleasure. There are many examples of this in royal and aristocratic circles throughout history.
Diana, however, was a romantic and idealist and of a different generation. She did not understand that Charles and Camilla were operating under established, unspoken rules. It went so poorly for everyone that it continues to negatively affect each and every day for the British Royals (see recent Harry comments, his coming book, etc.).
Had there been a previous aristocratic marriage of someone Shand-level to a future king, it would have been much easier. But previous kings had only married royals, with the exception of Bertie, who was not meant to be king.
Charles and the Queen eventually figured out that they needed to change things going forward, and now we have our first future Queen, Catherine, who was a commoner. In addition, Prince Harry had even more latitude than William and was able to marry an American divorcee, succeeding where Edward could not, three generations prior.
Had Camilla Shand been considered suitable at the start, things may have looked startlingly different.