Okay so what happens if I'm facing Yig, and holding the Serpent Crown, then before the crown of the serpent mystery is solved, my investigator is devoured, and I have to return the serpent crown to the game box? We treated it as an instant loss, but is there a stipulation for that?

Actually, when an investigator is devoured only their Investigator sheet and token are returned to the game box. All of their other possessions are just discarded. And the Crown of the Serpent mystery lets you search through the deck (including the discard pile) to gain the Serpent Crown. So you still could have solved the mystery, you just would have had to successfully gain another artifact to get the Serpent Crown again.

Okay, yeah I see that now. It's definitely clearer when it isn't 4:00 AM.

Thanks for the quick response

What if you can gain a non-item artifact. Can you gain the Serpent Crown instead, or does it have to be either an item artifact, or a generic artifact?

If the text says gain an <optional qualifier> artifact, then you may replace it with the Serpent Crown since the text of the mystery doesn't specify any restriction as to what type of artifact may be replaced with the Serpent Crown.