Thanks so much, Roy! This is a very big help. Up to now, I had no information on anyone named Beattey as part of the family.
The other thing that really excites me is that "Ishbella" is probably "Isabella." That is what Harriet named her eldest daughter, so I suspect it is one of the family names that has been passed down. In fact, stretching this as far as I can, it's possible that Harriet's mother was Archibald's sister, and was born "Isabella Beattey." Archibald's age also gives me a time frame to search for both him and my imaginary "Isabella."
I would greatly appreciate a copy of the passenger list from the Sheridan. Is it possible to e-mail it from the microfiche, or would it have to be mailed? If you let me know I can send a stamped, addressed envelope for it.