Can pregnancy and exclusive breastfeeding suppress endometrial growths and ovarian cysts?

No: Breast feeding doesn't suppress ovulation more than a month or two in most women, so it won't suppress cysts (most of which are follicles or eggs) and it certainly won't prevent ovarian tumors entirely. Obviously too if it prevent endometrial thickening most breast feeders who conceived would miscarry as well. Pregnancy does have some positive effect on non genentic ovarian cancer prevention.