Sorry, I have no date or place of death for Seth. He was living in Mansfield, Bristol Co. in 1776. He lived in Taunton to about 1733, them Attleboro until about 1748, and then Norton. Norton had been the North Purchase of Taunton and was established as a separate town in 1710. Attleboro had also been part of the North Purchase of Taunton, and had been set off earlier in 1694. It is evident from land records that Seth lived in the part of Norton that was established as the district of Mansfield, and that in 1775 was actually established as the town of Mansfield. Whether he died in Mansfield or not, I don't know.
As for his wife, Hannah Whittaker, I have seen 2 theories -b. 28 Mar 1705/6, dau. of Daniel and Whittaker and Mary Chafee; or, Hannah b. 19 Jul 1708, dau of Noah Whittaker and Sarah Read.
I do not havea Hannah b. 10 Sep 1736 listed as a child of Seth. I have 5 children.
As for William b. ca. 1645, I have no parents given.
My material is based on a 1931 publication compiled for the Briggs Family Assoc. I will be getting in touch with someone there (who has recently updated information on the more recent generations) to see if anything more is known about the early generations.
Do you have anything on Mehitable Blake? It's been suggested she may have been a young widow, and not born a Blake.