I don't have a definite answer for you, but I wondered that myself. With my miscarriage I had my progesterone checked at 4 weeks (as soon as I got a BFP) and my level was OK according to my doctor. Then had it checked again at 5.5 weeks and it went down so they put me on progesterone suppositories. I had u/s done at 6, 7 & 8 weeks and each one showed gest sac, yolk sac, fetal pole, but no heartbeat and no growth during this time. I stopped the progesterone suppositories at 8 weeks but didn't fully miscarry until almost 11 weeks. I asked my doctor if that is what caused the delay in my miscarriage and he said "no" that the progesterone is out of your system almost immediately. However, I wonder if I would have never started the progesterone in the first place if I would have miscarried sooner. I probably will never know the answer to that, and it really doesn't matter because if I had to do it all over again I would still use the progesterone. Mainly because if I would have miscarried and not used the progesterone suppositories I would have forever wondered if I could have saved the pregnancy by using them. I did all I could, and I know that.
I'm sorry I didn't really answer your question, but I just want you to know that I understand your wanting to continue using the progesterone until you have a definite and final answer. I'm so sorry you are going through this.