You are pretty safe unless they are deliberately exposed to a furnace or similar. The melting point of stainless steel is well in excess of the temperatures reached in a house fire or similar.

There's not point storing them together, just as there is no point storing your passphrase alongside your seed phrase. But given that there are more possible derivation paths (by many orders of magnitude) than there are potential seed phrases, it could be used as a "surrogate" passphrase, so to speak, where an attacker would need to know your seed phrase and derivation path. I wouldn't recommend it, though.

Fun maybe, but a quick Wikipedia search tells me the alloys which make up these coins have a significantly lower melting point and tensile strength than stainless steel, so probably not recommended.