I dont think the Group Seal was such a bad idea if it just fell out.
Before you go down the route of a new Collar, when you replaced the seal, did you take all the shower screen off again?
When I did a SSRC (Shower Screen Removal Clean) once on my 6910, it developed a leak after Id put it all back together again and I had the same concern. Just let it ride for a while and, when I did my next SSRC, the leak stopped after Id put it all back together the next time. I worked out that I just hadnt seated the shower screens back together correctly, which caused the leak.
So, my advice is to take it all apart again and fit it back together and that may fix your problem.
Re the black coasting stuff flaking off, its not of particular concern - I was reassured either AM or TG after I did my first SSR that its just a cosmetic coating and nothing to worry about particularly.