It wasn't just the forged letter that enraged the Duke, it was the simple fact he couldn't obtain any further money from the Duchess (and her father). Ian had married her for money, and when no more was forthcoming, he had to divorce her, much like he had his previous wives when their coffers also dried up. In Ian's eyes, Margaret (and probably women in general) were only as good as the money she could bring to the marriage.
Margaret's affairs were largely incidentally, and a means for him to divorce her, which he could do so once he had proof in the form of those photographs. Many of the supposed 88 lovers were gay men, and as homosexuality was still illegal at that time, Margaret didn't out them otherwise they would have faced persecution. I feel strongly that it was Margaret's enjoyment of sex that was on trial, rather than her adultery. The judgement rather delivered on this.
While neither were likeable people, it is easy to overlook that Margaret was a loyal friend to her Peter Combe and her other male 'walkers' and never told who the headless man was. Neither did she ever admit what 'v' meant. Imagine going through all this, having your sex life raked over by the courts, printed in the press all over the world and being vilified for enjoying sex, particularly oral sex. Yet she steadfastly refused to talk about any of it or betray others. To me, this shows a woman with at least a modicum of integrity, presence of mind and great discretion.