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If you stick the bimetallic coins into liquid nitrogen or whatever at these typical temperatures, the centres fall out due to different rates of expansion for the two metal alloys. It would work the other way round as well, by increasing the temperature to a few hundred degrees, but would increase the risk of surface oxidation and give the game away.

Sad to hear. As you will recall olive oil has an acidic pH and so not for lustrous copper coinage as you've found out.

As I understand it, none of these are genuine mint errors. The two parts of the coin are put together as blanks, before they have the design pressed into them. The minting machines would not be able to process separated pieces, so there is no way the patterns could be minted onto incomplete blanks. It is far too easy to knock the centre out of a coin and then claim "mint error" for a premium. Even worse are the ones where they have re-inserted the centre rotated or even back to front - again all impossible in the minting process.

If the coin is not high grade and is very dirty, then cleaning it is OK. But you need to be very careful with high grade examples. For high grades, dabbing with a cotton bud soaked in acetone should be fine, but don't rub, as this will cause hairlines.