Would Charles and Diana have divorced if Camilla wasn’t in the picture?

[MamoruHisaishi]

Was it really Camilla who ruined the marriage or was it just that Diana and Charles would have ended up divorced regardless if another woman or man was in the picture?

Camilla wouldn't have been accepted as a bride for Charles as she was considered to be promiscuous in her social circle.

I don't think she was considered to be (or actually) 'promiscuous'. She'd had a couple of boyfriends as most women in her circle would have had.

The main barrier to Camilla marrying Charles was that she wanted to marry Andrew Parker-Bowles. It didn't reach the stage of examining her suitability or the appropriateness of her sexual past for a Royal bride, because she didn't want to marry him, so the idea was never seriously explored.

It never reached that stage because he was sent away with the Navy. Remember communication wasn't what it is now.