I think I have some more clues on Seymour's parents.
See the following footnote/comment below in The Trowbridge Genealogy:history of the Trowbridge family in America (page 19 of 114): "!l One of the daughters of No. 119 may have married an Allen, for administration on the estate ot Trowbridge Allen of Aurelius, N. Y., was granted in 1814 to Jerusha and Seymour Allen. (See No. 154.)"
I believe that Seymour Allen's mother may have the oldest of the Issac Trowbridge's daughters, Naomi Trowbridge (bapt. March 26, 1749), especially given that Seymour's brother first name is Trowbridge and one of his grandson's middle name is Trowbridge.Naomi would have been about 19 years old at the time of Seymour's birth.This makes more sense than any of Naomi's younger sisters being Seymour's mother.
Please let me know if my assumptions make any sense to anyone else. I am still searching but hopefully this might clue us into who Seymour's father might have been and where this branch of the Allen family came from (maybe Conneticut?).