I looked for the death records for Jonathan and Margaret. They were obituaries in the Columbian Centinel which was published in Boston. Jonathan was in the issue of 14 June, 1820, noting he was age 67 (which fits his 1753 birthdate). Mrs. Margaret Seaver died in Boston on Tuesday before 5 April 1826, age 67. I have made the assumption that this is Jonathan's widow, although it doesn't say so. There may be a Boston death record for these folks, but I haven't found it yet if it exists.
The book I found is "Index to Obituaries in the Massachusetts Centinel and Columbian Centinel, 1784-1840," I don't have author or publisher. There is a similar index for marriages for the same dates. I'm sure NEHGS has it, San Diego public library had it.
The only child I have for Jonathan and Margaret is Catherine, born 6 March 1797 in Cambridge. You mentioned Elizabeth Curtis Seaver - do you have a birth date/place? Any other siblings? Is this the Elizabeth C. Seaver who married John C. Burt in Boston before 19 July 1817?
Do you know if this was Jonathan's first marriage? There was a Jonathan Sever of dorchester who married Priscilla Leeds and had children in Dorchester from 1774 to 1784l; Priscilla died in 1787. This Jonathan apparently died in Troy NY. My data says that this Jonathan was the son of Joshua and Abigail (Foster) Sever of Dorchester.
There was also an Elizabeth Curtis Seaver born 2 Oct 1797 in Roxbury to William and Lucy (Heath) Seaver of Roxbury, who married John Hayward 3 July 1823 in Roxbury. I assume that your Elizabeth is not this one. This was a granddaughter of Peleg and Patience (Curtis) Heath.
Neither Jonathan nor Margaret Seaver left a probate record in Suffolk, Middlesex or Norfolk counties.
Regards -- Randyrjseaver@home.com