So so sorry for your loss. It's such a tough thing to go through, and I know how much more difficult it can be when it's drawn out.
I'm not sure how helpful my experience will be; after my first m/c I had a d&c and bled off and on for weeks after the procedure. They never tested my beta, but it took a really long time for my body to normalize. With my second m/c, my doctor told me that if I took the drug it could take a few weeks for my hormones to level out because my beta was so high, and because I had not had any bleeding on my own. (I ended up having another d&c for other reasons).
Can you ask your doctor at what point they will want to do or check something else?
Again, I am really sorry you're going through this. It's so difficult.