I don't know about the Welsh Allyn connection, but I do have a question about Isaac Allyn. My husband has an ancestor, Mary Kimball Allyn, born in Preston, New London, Conn. in 1795. Her parents were Ebenezer Allyn and Polly Kimball. Mary Allyn was first married to an Isaac Allyn. I do not know if that was in Connecticut or in New York, where they were later found. I know they had one infant, who died in New York. Mary's father, Ebenezer, also died in New York, and he and the infant are buried side by side. Mary and Issac, along with her mother, sister and three brothers, then moved to Feliciana Parish, Louisiana, along with other families from Conn. In 1830, Polly and Isaac were divorced, and I have never found him in another census record. I don't know what happened to him. If you, or anyone else reading this post, can shed any light on this family, I would love to hear from you.
Sue