All stainless steel plates with bolts I saw in his testing tended up performing poorly compared to regular single plate without bolts.
Most other designs which include bolts or screws are designs which utilize the "tiles in a holder" type design, which I agree perform very poorly as even a small amount of warping movement and all the tiles fall out. This design doesn't feature the same drawbacks, and if the screw or bolts loosen up a little here then no data is lost.
On the other hand, if I make mistake with Coinplate I will have to throw it away, order new one and wait for shipping.
I don't think so. If you mark the wrong letter it would be pretty trivial to hammer in a few more marks to obscure that letter to signal you made a mistake and instead mark the correct one. You would have to make a large number of mistakes for it to be completely unusable and require an entirely new device.