Yep! Cheap pads cost me way more than the good ones ever did.

The glaze peeling around the chip isn't from the salt scrub, that chip is what broke the seal. Once the glaze is cracked the clay underneath soaks up whatever you put in there and it flakes off from the inside. Salt can't eat through fired glaze, so you didn't wreck it, it was already done for. I'd retire it, you can't really seal exposed clay on a mug you drink from.

Pilot holes should match the shank, not the threads. That is the whole game right there. I did the same thing for years and could not figure out why half my screws kept snapping. Once I got a bit set with the sizes printed right on the case, everything changed. Marking depth with tape is a nice touch too.