I wonder if you've found Alexander's and Lydia's grave sites, yet? If not, it's possible that they are buried in the Stuart Mountain Cemetery in Machiasport. I was there in October 2001 and found gravestones for their son, Alexander, dau-in-law, Eunice, and granddaughter, Hattie, along with several others of Alex & Eunice's children, son/dau-in-law's, and grandchildren.
Unfortunately, the graveyard, at that time (and possibly still) located on property owned by Mr. Mike Hinnerman, was in pretty bad shape with several stones worn, laying on their backs, sunken, and very possibly missing. There's a good probability that Alexander and Lydia were buried there, as well, especially since it's called "Stuart Mountain Cemetery" and the Stuarts buried in it are from Alexander and Lydia's line.
I am also researching Stuart/Stewart families from Washington Co (specifically, but not limited to the Machias/Machiasport area) and would love to share and compare information with you. I look forward to hearing from you and any other Stuart researchers of the area.
Quartz Hill, CA