Mary, I wanted to say thank you for bouncing this back and forth with me. I find it helpful to help clarify my thoughts. Like you, I have no horse in this race. My connection to the Lowe family is VERY distant. My ex-husband's 2xgreat-grandfather's sister was Laura D (Meares) Lowe, Capt John T Lowe's wife. Here's the latest installment.
The Jr part doesn't hold any special consideration for me because my experience has been that Jr is just as often used for the younger of two totally unrelated men of the same name as it is for father/son. The mariner/sailor/seaman notations for occupation also don't hold a lot of weight in this area as we're talking Monroe County/Key West and immigrants from the Bahamas. The majority of the population do something regarding the sea.
I am concerned that there might be more than one couple named William Lowe and Mary Russell. The names are so common and the Lowe and Russell families were prolific in that time and place.
I found a marriage record for a John Lowe Jr and Mary Elizabeth Lowe in 1856 which explain why Mary Russell Lowe would be with them in the 1850 and 18970 Censuses. She could be the mother of John's wife. Mary E is also referred to as Lizzie. Her age on the various censuses is consistent with that of William and Mary's daughter Elizabeth as seen on the 1850 Census. Of course, no parents identified on the marriage record so still no confirmation:
Florida, County Marriages, 1830-1957, database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FWW7-Y9W : accessed 12 January 2017), John Lowe Jr. and Mary Elizabeth Lowe, 09 Dec 1856; citing , Monroe, Florida, United States, State Archive, Tallahassee and clerk of courts, various counties; FHL microfilm 946,962.
Mary Russell Lowe living with her daughter and son-in-law seems more likely than living with a more distant relative.
I know a couple of the Meares family descendants who I'm going to contact them and see if any of them have any probate or church records that might provide the smoking gun. I'll keep you updated.