Hi LouLou,
I'm really sorry you went through that and for your loss.
I just thought I'd add my experience as it's been a real area of confusion for me, too.
I miscarried at about 12 weeks at the end of May. It took a lot longer than the three weeks I was advised for the pregnancy test to be anything less than a blazing positive. It was still strong at five weeks while we were away on holiday, and the hospital advised we go back to them when we got home to the UK. Anyway, my first period did then arrive at about six weeks, and although it's now finished I am still getting positive tests - it is definitely getting lighter, but still a definite positive.
Two weeks before my period arrived I had very strong ovulation pains - that and the length of time after the miscarriage make me think it was a real cycle and period. Given there is still HCG in my system, I would say from my own experience that, yes, you can ovulate and so, I would assume, get pregnant before HCG levels reach zero.
Before my period arrived and I was still in the limbo stage trying to work out where my body was at, I did a lot of researching to try and find the answer to your question, too, but didn't find anything very conclusive either way... I suspect it's one of those "we're all different" things, but I hope that might help a bit.