It sounds as though you have had more problems with your db marriages than I! I think you may be right when you say it might be best to download your db from Ancestry again.
When I was trying to sort out my problems with incorrect marriage dates showing I also discovered a few phantom spouses. I eventually managed to get rid of them but can't be sure exactly how I did it. I have a feeling that I unlinked all children from the parents first, then unlinked the father(s) and mother(s) from each other before "rebuilding" the marriages and families. Might be worth a try, if you don't want to go to the trouble of downloading your entire tree again.
I have also noticed that my marriage list has quite a few missing MRINs - presumably these relate to the marriages I had to delete to get round the erroneous display problems. The missing MRINs do not trouble me unduly though I did wonder whether this might be corrected if I were to download my tree from Ancestry to a new db. To be honest, I'd far sooner put up with a few missing MRINs than have to deal with people apparently marrying long before they were born.
For what it is worth, after sorting out my earlier problems I don't go anywhere near the TreeShare button until I have carried out my new marriage routine. First, I enter the spouse and marriage fact on Ancestry and check that this has been correctly applied to both parties to the marriage. I then add the spouse and marriage fact in RM. Only then do I dare to use TreeShare to synchronise RM with Ancestry. Since I adopted that procedure, I have not noticed any erroneous marriage data being displayed. I hope I haven't spoken too soon!