An update. I am learning as I go.
The slightly "wrinkled" bullets I thought might be polluted with zinc were sized and then weighed and are right on spec. And they are relatively soft, virtually identical to previous "clean" batches. I suspect I was casting at way too high of a temp, as some of the wrinkled bullets dented/ deformed when they hit the soft board tabletop i drop on to (yes, i dont drop into water). They were evidently still semi molten coming out of the mold.
I couldnt bring myself to discard 50# worth of ingot, even though I was sure it has zinc in it. I did not want to get into sulfur, so i tried again with the smelt pot but at a much lower temp. It took a while to bring it to a molten state, but then I immediately reduced the flame once it had melted down. As I stirred, quite a bit of silver-grey oatmeal formed at the surface. After quite a bit of diligent skimming, I had quite a pile of zinc-like material in the discard pile. Probably 5-7 lbs. Crunchy brittle oatmeal. It is nowhere near as heavy as lead would be, so I am confident in what I removed. As a test, I cast about 50 swc from the pot so i could hardness test. Damned if they were not the most beautiful silver. Weight is correct. Hardness test in the vice was spot on. The rest of the pot i cast into ingot cupcakes, and they are the right color, no galvanized sparkle, and they "thud" on the floor, not bounce. yes i drop the cupcakes and listen/watch. I do need a life.
I consider the evening a success. And some lessons learned: more heat is not better, avoid mystery metals, wrinkles are the fault of the person casting, not the metal.