Why can Prince Charles still inherit the throne when he married a divorcee like Edward VIII who was forced to abdicate when he did this?

Edward was not 'forced' as such. He could have married Wallis Simpson and kept the throne if he really wanted to. The problem is, he knew there was major opposition towards this, and that's why he chose to abdicate.

Consider also that he was the reigning monarch at the time. In Prince Charles' case, he was not (and of course, still not) the country's monarch. His marraige to Camilla Parker-Bowles is a civil one only. His ex-wife died long before the marraige, but Edward's was still alive. Any children they would have together would not be included in the line of succession - the throne would be going to Prince William after Charles anyway.