I am so sorry that you are having this bleeding and also that you have to wait so long to find out the cause. I am quite a bit older than you and am post menopause, but I had a bout with bleeding a couple of years ago brought on by a pessary that was inserted for my prolapsed bladder. If you don't know what that is, no need to know. But briefly, any of the pelvic organs can drop down due to weakening of the ligaments that hold them up. In my case it was the bladder and my doctor had inserted a pessary which is device to lift the bladder back up to it's normal position. It turned out that the pessary was rubbing and causing my bleeding. Since you had the IUD and now it has been removed and the bleeding is tapering off, it sounds as if the IUD may have been causing some irritation and thus bleeding. With the polyps and the IUD, it could be the IUD was causing rubbing or irritation to the already irritated situation of the polyps. In my case, it took quite a while for the bleeding to taper off after the pessary was removed. And I had to be very limited in my physical activity or it would start up again. I ended up having my prolapse repaired because I could no longer use the pessary. And I have had no bleeding for a couple of years now. I draw the parallel here simply to point out that it can take quite a while for bleeding to stop even after the irritant (in your case possibly the IUD) is removed.
Seems as though they are not overly concerned due to the results from your biopsy showing no malignancy. So that hopefully can help you to think positive. I am not a doctor but I think if they thought it was terribly serious you would not be waiting two weeks to get back in there. But still I understand that the bleeding is scary and annoying as well as concerning when you don't know what is causing it. I would not hesitate at all to call back if you begin to feel the slightest bit weak, light-headed or otherwise ill. I had a history of anemia prior to my bleeding and so I had to be very careful and monitor my situation. Once you do figure out what is causing the bleeding and once it is stopped, I would follow up with some blood work just to check and make sure that you are not anemic or having any issues as a result of the bleeding since you said you have had irregular bleeding for a while now.
Please take care of yourself and keep us posted on what you find out. You are doing the right thing to gather as much information as you possibly can from this and other sites.