I have a bottle of 12mg and one of 16, same flavour, that I've been mixing together from the start. It's hit or miss, which I rather like. Have another shipment of different flavours coming soon, that will add more variety.
I think if I had a used cart sitting around for longer than a day, I'd swab it out. That's just me being me, not me being knowledgable. And for the swabbing, not seeing a lot wrong with a touch of soap on a q-tip, plus warm water and rinse and air-dry.