The Ft. Seybert booklet has not convinced me that Oliver's letter is wrong, rather that it is strongly disputed. I would really like to see any documentation for Olivers claim. As far as I can tell, there is no documentation that Josephus existed. No birth records, court records, census records or anything.
The burden of proof is on the party making the claim. I am not 100% convinced that Oliver's letter is wrong, but it is my opinion that the documentation and references of the Ft. Seybert booklet is so extensive, compared to Oliver's letter, as to make it unlikely that the claim in the letter regarding Josephus Coonrod is correct.
As you told me yourself, you have spent 15 years attempting to prove this claim. Is there any record to support Oliver's version of the Massacre story? Is there any documentation that even places Josephus Coonrod at Ft. Seybert? Is there any evidence to dispute the Dyer Family version?