I would say that it's very feasible that Abraham, Sr. died in the Susquehanna Valley, Susquehanna, Pennsylvania, in the fall of 1745. The probate hearing took place nearly a year later. It probably took awhile for news to reach Jemima, because she died in 1946 (probably of a broken heart). The Susquehanna Valley (Quaker country) is near or adjacent to Bradford, where grandson Isaac visited prior to 1798, before settling in Arcadia Township, Wayne, New York, with brother Samuel, Jr.
The confusion regarding the records is that there is no distinction between parents and children (i.e. Sr. and Jr.). Therefore, it's quite feasible that it was Abraham, Jr. who acted as witness to Rev. Daniel Taylor's will.