yes your levels are still high after the d&c as if you are pregnant. thats why after a d&c you kinda bleed because your body is still getting rid of the baby out of your system.
I think it is different for everyone. I took a hpt a week and a half after my d&c and got a very faint positive. I believe (due to the positive OPK I got) that I ovulated exactly 3 weeks after my d&c. I got my first period exactly 5 weeks after my d&c. I was only 7 weeks pregnant when I had my d&c. I am sorry that I don't know the answer to your question on if the HCG has to be all the way gone to ovulate. I am guessing it atleast has to come down and be very low.