well i agree that in the Royal world, unfair as it is, her being engaged in an affair or affairs would be seen in some ways as "worse" than Charles' doing so.. He was male and the royal partner.. She was only married into the RF and of course there are double standards. But I think that to say her behaviour was "inappropriate" is very unfair, if his isn't being seen as inappropriate.
But I don't know if Diana was really so calculating. True she didn't tell about her afairs until later on, DHTS focussed on charles' faults as a husband, selfish, unfeeling, unfaithful, etc.. and Di's own feelings that the RF were also cold and over formal. but then in an autobiographical work, one doesn't usualy lead with one's faults.
I think that chalres only addressed the affair with Camilla in his biography because he really HAD to..
I just don't know if Diana was really calculating in "doing" Diana her True Sotry. I can't help thinking it was partly an attempt to knock Charles, partly perhaps an attempt to get a divorce and partly just a confused painful lashing out, without a coherent plan.