I have looked long and hard in my Seaver database (mainly descendants of robert of roxbury) and do not see a positive connection to that descent.
However, I am very confident that your Joseph Seaver who married Betsey Davis IS from the Robert Seaver line, either through the Kingston, Bridgewater or Taunton families. Unfortunately, the records are poor in Bridgewater and Taunton.
I checked the probate records I have (some wills, only) for Plymouth and Bristol Counties and didn't see this Joseph mentioned. But, he may be in the probate packets for someone who didn't write a will. This will take more research to clear up.
In the meantime, would you mind sending me what you know about your line? My new eddress is firstname.lastname@example.org
Regards -- Randy