Dear Mamallamaa: What An_6061 has raised is true (as usual), but I would like to add some additional input. Yes, an 8 mm thick uterine lining could be seen with birth control pill (BCP)use. A BCP exposed lining can even be 3-5 mm.
Hormone levels taken when one is on BCPs may not be very useful. The estrogen blood tests routinely used are for our ovarian estrogens--not ethinyl estradiol (estrogen used in BCPs). So the test will show very low estrogen levels in BCP users--despite all the estrogen she is taking (doses in BCPs are 5-7 times higher than in HRT).
Testosterone will be lower in BCP users, too. The estrogen in a BCP stimulates the liver to make more binding proteins (SHBG). SHBG binds testosterone making it inactive, and drives down testosterone production. This is why BCPs are used for acne and unwanted facial hair (testosterone side effects).
OK, to answer your questions. I would cast my vote with the two MDs that said "D&C not necessary". BCPs contain both estrogen and synthetic progesterone. Simply stated, if the uterine lining is like grass or lawn, estrogen is like the fertilizer (causes a thickening of the lining ), and progesterone is like the lawnmower (keeps the lining thin by three different mechanisms). This is why DepoProvera (high dose synthetic progesterone) brings about a thin lining, and why BCP (relatively progesterone dominant) bring about shorter, lighter periods. BCPs were designed to be progestin dominant to prevent the lining getting thick.
Finally, some women on the newest low dose BCPs do get some vaginal dryness. This can be improved by changing the brand or formula of her BCP. If you are still on a BCP, I'm not sure I would add additional estrogen or testosterone creams--just a personal opinion (which I rarely share here). An_6061 suggested an endocrinologist -type evaluation. That is a reasonable suggestion.
Yours,
Jane