I'm sorry you're having this problem. I don't have any experience, but I can try to give you the risks and benefits of each option.
The pros of waiting are that if the peices come out on their own, you avoid having drugs or medical procedures. The cons of waiting are that you risk infection.
The pros of medication are that if it works, you avoid the risks of surgery, but have a better chance of getting the retained placenta pieces out than with waiting. If it doesn't work, you'll have to have a D&C anway (one source I found said this occurs about 3% of the time, see http://dandcnow.info/miscarriage.html). Misoprotstol can be dangerous for induction (increases risk of uterine rupture during labor), but it appears to be pretty safe for retained placenta and missed or incomplete miscarriage (if anyone else can find more info on this, feel free to share, I can't find much about it).
The benefit of D&C is that it should take care of the problem right away--no waiting or wondering if all the pieces are out. The risks of D&C (from the article I linked to above) are that it can potentially damage your uterus and cause scarring, as well as all of the risks of surgery.
I don't have any experience, but I wanted to send you (((hugs))). I agree with Brittany, that miso does appear to be fairly safe when used post-partum according to the studying I have done on it. How much longer would your provider recommend waiting to see if it will pass?
If it were me, I would probably wait a little longer and see if there are some natural supplements (like you are doing), then try miso, then last-resort do the D&C if it came to that. Does your MW have any natural remedies? Or maybe contact a naturopath? I hope everything works out for the best!
Are you experiencing any complications from the retained placenta? Is your milk in?
At 7 weeks pp I would probably opt for the miso myself. Less risk than a D&C...and at 7 weeks pp it seems unlikely that you body will expell it itself, but that's just my pov.
I'm sorry you're dealing with this! It would be nice if we could have our babies and not have to deal with issues like this afterwards when we should be worrying about other things.
At this point, I'd go for the D&C (although they usually use suction now, called vacuum extraction, instead of a curette, which is like a little spoon-shaped scalpel) for two reasons: first, the miso is going to give you horrible cramps, and why put yourself through that after all you've been through already, and second, you know it's over & done. A vacuum extraction isn't surgery, it's done on an outpatient basis and requires only a local anesthetic on your cervix. I had an abortion many years ago, which is exactly the same procedure you would be undergoing, and physically it was quite literally a breeze. Over & done in less than 5 minutes, very little bleeding, no cramping, the little needle prick on my cervix was the worst part, aside from the emotions involved, obviously. It's really not an awful procedure to go through, and the risks with the suction method are much much lower than with the curette. If your doctor will be doing a vacuum extraction, I'd absolutely do it over miso. If he recommends a surgical D&C, which requires general anesthetic, I'd first ask why he thinks that is better than a vacuum extraction (and I'd suspect insurance billing might be a reason!) and if he holds firm then I'd choose the miso, or find another doctor to perform a vacuum extraction. Good luck, that sucks to have to deal with this now.