Hi Alison, and welcome. To go back to your original question: when you add metabisulfate to the must should you hammer down a lid to keep in all the sulfur dioxide or do you want that chemical to dissipate fairly quickly. In my opinion, you want the latter. While commercial yeasts are cultivated to be rather less affected by SO2 compared to indigenous yeast they are still sensitive to that bactericide. Indeed, we add SO2 each time we rack and just as we bottle, part to inhibit oxidation but also to inhibit wine refermenting in the bottle. That concern is especially relevant if you are planning to back sweeten.
You want to make sure that when you pitch (add) your chosen yeast there is really no SO2 hanging around to stress the yeast. You can do that by allowing the gas to dispel over 24 hours but it can do that only if you provide a large enough opening for the gas to escape. Sealing your fermenter is not usually the best way to handle the SO2 produced.
