Sammiejane, I can not give you the confirmation you seek regarding progesterone as a solution for your hairloss. Exogenous estrogen like Estrogel doesn't act the same way as our own estrogen or our hormones. It doesn't mean Estrogel is causing your hairloss, it could be you need more or a different kind of delivery like patches. The fact that you didn't experience hairloss after your hysterectomy could be because your estrogen levels didn't drop instantly but gradually decreased. Your blood test shows your estrogen is low so that's given you some indication. Low estrogen can cause thinning hair. I think your specialist is right when he wants your estrogen levels to be higher before adding testosterone.
Again, I understand you are frightened but the only way you'll find the answer is by making a choice either way as no one can say for 100 % sure what's causing your hairloss. It remains trail and error. If you feel so strongly you need progesterone or a BCP, as I feel you do, you should give it a try.