I have also been looking into this mystery for years. The family lore has always been that Silas died shortly after Clara but there is no evidence of any kind that he did die in 1890, whereas there is plenty of evidence that Clara did. I have found no death record for Silas and no grave records. I can find no records to establish guardianship of his children and no probate records. It all just seems so strange.
I contacted someone at the Solon Historical Society and she made a few inquiries. It seems the Cemetery records from before about 1910 are lost. She also had someone use a pole to probe the grave and he could not determine if there was a second burial in the plot.
I am trying to get in touch with a woman who is descended from Silas' brother Asa. She seems to know a lot about the old history of the family, but she moved recently and put off contact until she is settled. Up to now my contact with her has been through layers of go-betweens. If I hear more from her I will let you know. She is also of the opinion that Silas' mother lived well past 100 but I also cannot verify her place/date of death.