No outsider can know what intentions Charles had. One can't simply believe Diana's version because she couldn't know it either.
There are two basic questions. First, did Charles and Camilla have a relationship in the first years of his marriage? As far as I know, there is no evidence that they had.
Second, is there any reason not to believe Charles when he said in the interview with Dimbleby that he was unfaithful only after his marriage was irremediably broken? Has he ever lied in public in any other matter? Diana, on ther other hand, isn't known for her truthfulness.
Of course Charles was wrong to propose, but he believed in the image Diana presented him in order to catch him. She intentionally pursued him and showed just those characteristics which she know would impress him. When she in their first meeting said how sorry she was about his lonely grief over Mountbatten's murder, with nobody to comfort him, she intentionally gave a hint that she was that kind of girl who would do so.
And Diana wasn't just like any girl of 19 years but exceptionally naive, uneducated, full of romantic fantasies, scarred by her parents' divorce, with no plans for study or work, yet ambitious to marry well.
Of course Charles did wrong, first not to present Camilla as his ex, and then when Diana found out, to think that she would be satisfied with his assurance that it was past, yet still showing his feelings by using Camilla's present. But he did try to help Diana adapt herself to her public role (when she f.ex. refused to go put from the car) and get the marriage work.
Would a normal girl, however disappointed in her marriage, really have reacted like Diana (mood swings, bulimia, incising, suicide attempts)? Probably she couldn't help herself, when even medicine experts hardly understood these kind of physical and mental illnesses. But how many husbands in the world would have endured the situation in the long run?
That one does see also Diana's weaknesses doesn't mean that one denies her positive qualities, on the contrary her exceptional ability to connect with people who had experienced hardships was probably born in her own experiences.