The buckets full of stuff are probably the accelerants - the things they need to take the gummy polymer base and turn it into cross-linked rubbery stuff. "Buckets" would be a pretty big batch - I watched a 20kg batch of Viton get mixed up last November, and they used about a quart-sized plastic bag full. It got mixed up in a screw machine mixer, it was essentially a 3 foot diameter motor (50 hp+) directly attached to a pair of wide paddle screws in a little mixing chamber, and that was the whole thing. The amount of power density in that machine was staggering. The rubber mix would cause the whole unit to bounce and shake while it was being churned.
After that, it gets rolled out into sheets and sliced into preforms and used. You are correct that at this stage room temperature storage can start the curing process, so it typically will get used within 1-2 days, if not immediately. Viton/FKM does need post cure, but I've never heard of a microwave oven to do the job, it's usually a large forced air convection oven.
Also, once the molding process is done the rubber is permanently, fundamentally changed on a chemical level, so if any of the molded scrap is recycled and reused, that is a serious cut-rate manufacturer and should not be trusted for any kind of sealing devices. Scrap from the preforming process when the uncured material is rolled out into a sheet and the sheet is trimmed can be re-mixed, but that is usually kept to 10% or less at reputable places.