Wow, if you're piston-whacking the plugs, you almost definitely have the incorrect ones. If you're sure that you have the right plugs, look for signs of detonation/pre-detonation. Prolonged detonation causes enormous stress on lots of things, including plugs, and could easily cause bent electrodes. Notice any knocking/pinging? You say that you're a tooth off timing-wise, I'd work to rectify that before all else. Having jumped time won't precipitate the piston striking the spark plug(s), but could cause detonation if the timing is advanced by skipping a tooth, though I would think if you "jumped" time it would be slightly retarded, not advanced. My Z24 is an 8 plug engine, I assume yours is the same?