Nice bit of sleuthing!
On the Japanese bikes the metallurgy seems to focus on surface coatings over mild (i.e. cheap) steel. Once through these, the part is essentially "gone". Is it the same thing on these Electra cams/lifters, or is there real metal to be had there?
Did the driver appear to be a dodgy lifter from the start, not spinning properly and so eating away at the cam nose? Or would running the valves too tight possibly affect lubrication of this cam/lifter area by not allowing a proper oil film to form, maybe initiating a cavity on the lifter rubbing surface?
Leveling out the lifter with the Dremel and fine wheel should stabilize things, but the "fix" may ultimately be new cams & lifters. There's no substitute for cubic dollars, but every rebuild runs into a owner-imposed money limit. At least this can be done in the frame when the owner's (or next owner's) bank account recharges.
Thanks for taking us through your troubleshooting process. Always something new to learn! Regards - ACR -