Well Mary junior is not a problem. You know her full life story. Person I mean is David H. Lowe's sister. You have her obituary "Mary Lowe Hanson Merrick." You have photo, her death certificate. You have a record of her in the census in 1850 and 1860. I haven't looked for her in 1870 and 1880 and onward. Her obit mentions the names of her husbands and the names of her children and where she lived. I guess to me, I felt like that was enough - better than a census, which tells so little about the person. :D
You wrote "Right now I lean more towards Mary Sr. as being the grave site that you found." Yeah it might be. I agree it's good to keep an open mind. Doesn't hurt to have a "possibility" to be checked out.