Does Amazon commingle third-party stock with their own, or are their vendors selling them fake chargers?

When I was looking at selling my creations on Amazon, they had an option to commingle stock with other sellers of the same item, but I don't know if that includes Amazon's own stock.

Yes. They have a webpage somewhere describing this. They do not commingle select categories, such as food. Everything else is fair game for commingling. Sold By Amazon is no guarantee of authenticity or even an indicator that your product came through a legitimate supply chain.