Carol, I think you're really on the right track to find the children of William and Mary Lowe, in order to find Mary's maiden name in records for them.
The records posted to date for them by Miles show them in 1850 in Mount Pleasant, Green, Wisconsin. Their youngest daughter is Mary E. Lowe, age 1, born about 1849. Then in 1860 Mary age 53 is a widow living in Greenfield, Monroe, Wisconsin. Daughter Mary E Lowe is age 13, still the youngest child in the family. So Mary Merrick cannot be the daughter Mary in this family; born in 1888, she is too young.
In the 1850 census in Green County, Wisconsin, a Thomas Sanderlin lives next door to William and Mary Lowe. Then in 1860, a "Susan Sandelin" age 8 is living with Mary Lowe and children. I am very curious to know who is Susan? Who are her parents, and are they related to William's wife Mary? I haven't pursued this line of research and will leave it to anyone else who can do it (I hope you'll pursue it). The proximity of "Sandelins" to Mary in the census suggests she might have maiden name of Sandelin.
1860 Greenfield, Monroe, Wisconsin Post Office: Tunnel City Mary Lowe 53 born about 1807 Pennsylvania William H Lowe 20 John C Lowe 17 Cyrus L Lowe 15 Mary E Lowe 13 Susan Sandelin 8