Two steps forward in my search, at least it feels like forward motion, came in the form of emails. The first was from the County of Ontario in New York and their RAIMS office. Wondered for about 20 minutes what RAIMS was and then went back to the home page to find out that it's the Records Archives and Information Management Service! OK, so I can see them wanting to say RAIMS and not that whole mouthful!
Rosemary at RAIMS emailed back to my query that yes, they did have both a deed and probate records for a Nehemiah Newan in the target time period. Fill out the form and send $25. DONE. I think that the probate records indicate he died in 1820. Hopefully that will be cleared up when I receive what Rosemary found. I'm clueless as to what the deed might be about.
The second email came from that nice Mr. McWilliams of the PA Archives. He gave some handy info about searching for deeds in western Pennsylvania counties. Here's what he wrote: "Keep in mind, District 1 lands were located in Westmoreland County in 1785-1788, but would have been in Allegheny County, 1788-1800. The lands are currently located in Butler and Lawrence Counties."
Petition for PA Land grant Page 1, 1818.