I personally believe most of the confusion is from the illegibilty and mistranscription of census records.
The earliest reference I see to the wife of Solomon Fantz as being Susannah 'Sefler/Seffler' is from the 1860 census for Marion Twp., Hocking Co., OH. The handwriting of the census taker does indeed resemble an 'S' as we know it today, but if one scrutinizes other surnames written by the same census taker on preceeding pages, one can easily his 'S's' and 'L's' look similar. Also on that census listed as living with Solomon and Suannah, is Susannah's mother, Susannah (nee Showalter Reedy Leffler), 65 yrs. old b. VA.
Aside from that, my husband's 4th maternal great grandmother, was Rebecca Leffler, daughter of Peter Leffler Sr. and his first wife, Catherine (surname unk). Peter Leffler's 1st wife, Catherine died around 1841 and Peter remarried to Mrs. Susannah nee Showalter Reedy, some 20 years younger than he, on 6/11/1843 in Fairfield Co., OH.
Mrs. Susannah nee Showalter Reedy, widow of John Philip Reedy, had at least 4 children that bore the last name of Reedy, although some Reedy researchers claim John Philip Reedy died around 1828, which would mean that at least 2-3 of Susannah's children might have been illegitimate, including Susannah that married Solomon Fantz.
The documents that I have copies of that lend weight to that theory (as of yet, it is JUST a theory) are the death certificates of two of Solomon and Suannah Fantz's children, Jacob L., and Absolum Fantz, who both have their mother's surname listed as 'Showalter' born in VA.
Hopes this helps to shed some light.