Going back to the c-section aspect - it used to be done down the abdomen rather than side-to-side. That wasn't so good for the mother, because the incision went through the circular bands of uterine muscle, which raised the risk of rupture in subsequent pregancies. Whichever way it was done, it must have been rather risky for the Queen, as she gave birth to Anne just under two years after Charles.
Maternity fashion is quite interesting, because it's influenced by contemporary thinking. Corsets of the lace-up kind would have accommodated the growing bump, but certainly even when I was young it wasn't seemly for a bump to be obvious. By the time I was pregnant, in the 1970s, Mothercare was the place to go. I bought a wonderful pull-on that was stretchy but had a heavy supportive band that went under my bump. (Didn't completely stop the bouncing on my bladder, though!)
Mothercare back then had good tops that were designed to go over the bump without acres of cloth. I did think Diana's choice of maternity dresses very frumpy, or maybe she was using that style to make her pregnancy obvious. I'm pretty sure she could have gone for a much more tailored style.
As for Kate not using pain relief - I think that a lot of people equate pain relief with epidurals. As she was quite clearly able to walk just a few hours after giving birth, the assumption is of no pain relief. But there's lots of other pain relief that wears off quite quickly.
Somewhere here I've got a book on royal births. I might try to find it.