Does sufficient capacity for recasting actually exist in the US or would it need to be built? If the latter, how long will it take to build it?
I suspect it doesn't exist yet. In fact, the majority of the US trade deficit with Switzerland is attributable to a very recent spike in gold imports. And my understanding is that in turn is a triangular arbitrage trade where physical gold has higher prices in the US, so 400-ounce bars of gold were getting exported from London to Switzerland to be recast, and then to the US.
This was never going to be a durable trade, since the arbitrage only existed due to an expectation of future tariffs on Gold. It probably also was a major contributor Switzerland having just been slapped by extremely punitive tariffs on actually productive sectors of the economy. Good riddance.