Oh gosh - I'm sorry this is so complicated. Miscarriages are difficult on their own without the complications! I would definitely consult your OB, not just the ER. I know they can wait quite a while to let a miscarriage pass on its own, and I think hcg can vary a bit if the tissues are slow to break down, but at some point it really will be advantageous to have the d&c if the miscarriage does not complete on its own. I had a later miscarriage and had a d&c with it, and it was really fairly simple, at least physically speaking. Best of luck to you. I hope everything goes as smoothly as possible from here on out!
You said the ultrasound showed a solid mass, but they "think" it was a blood clot if I were you I definitely would want to make sure they knew what they were talking about. Please keep me informed as to what is happening with you. If you're HCG levels don't go down, make them pursue it further.
There is a very rare disease that can happen called gestational trophoblastic disease, like I said it is very rare but very serious. My sister had it they couldn't understand why her HCG number wouldn't return to 0 but was elevating. The fetus/tumor had dislodged and they couldn't get a handle on the disease.
I am not trying to scare you and I am so sorry if I am but I just wanted to keep you informed. It took them months before they figured out what was wrong with her.
I hope its just that you might need more time or a d and c to help your body along. I will hope and pray that everything works out for you.