Over the past eight years we have observed optical spectra of planetary nebulae (PNe) in the disk of M31 using DIS on the 3.5-m ARC telescope at Apache Point Observatory and OSIRIS on the 10.4-m GTC on La Palma. We have so far studied more than two dozen objects over a projected galactocentric radius range from 5 - 33 kpc; this corresponds to a deprojected in-disk range of 15 - 106 kpc. Using ELSA, a five-level atom package, we have derived nebular diagnostics and ionic and total nebular abundances of He and O, as well as estimates for other elements. The average 12+log(O/H) for 23 disk PNe we have observed is 8.6, or about 80% of the solar value. The inferred oxygen abundance gradient across the disk is surprisingly shallow (~ -0.004 dex/kpc) out to R(deprojected)~60 kpc. CLOUDY models we have computed for many of these objects indicate central star masses whose main-sequence progenitors are estimated to be in the range of 1.7-2.5 solar masses, with lifetimes under ~2 Gyr. The existence of such young, relatively massive, and metal-rich stars past the outer edge of the spiral arms at ~18 kpc and the H I warp at ~30 kpc (beyond which stellar [Fe/H] < -1) is unexpected, and disagrees with standard models of outer galaxy assembly via assimilation of metal-poor dwarf galaxies. Star formation from inner-disk ISM ejected by a putative gravitational encounter between M31 and M33 about 3 GY ago (Bernard et al. 2012, ApJ 420, 2625) supplies a possible explanation.