Chaun Mortier at TruckeeHistory.org has responded via email with the following:
The first question, yes he was married to Margaret Ann Newkirk, she is buried with him in the Cemetery Family Plot. The year of death is not wrong, he died in 1891 and the entry you refer to has the date of birth at 1834, this has been confirmed through descendants of Jacob. I do appreciate your attempts of keeping history correct. His first wife Rhoda is not buried in the Truckee Cemetery, that I believe has been a "wishful thinking" issue. If anything she was buried in the first cemetery that has now been built on or her family came and took her back but in his letters, after her death, he never mentions where she is buried.
I am the archivist/recorder for the Cemetery District so I can assure you his death date is correct but when it comes to things on headstones I have found many, many errors and they were made by the Monument maker. His date of death is very hard to read on his monument but the data in the entry is correct.