I should clarify that point about hardeners.
Yes, alums are faster in acidic medium and the reaction can be reversed. They harden so slowly in some cases that an alum hardener can be stored within the emulsion for long periods before coating.
Aldehyde hardeners are rather fast but slow enough to keep hardening after the coating is made. They can fog an emulsion if used to excess. They cannot be stored in the emulsion during keeping before coating. They react more rapidly in a mild alkaline environment.
Newer hardeners are extremely fast acting and usually cannot be mixed with gelatin at all. They are often applied in a separate operation, but once coated they are "done" and the coating is hard. There is no after hardening and there is little pH effect. In fact, these hardeners are often stored with inhibitors to prevent them from being too rapid. Many of them are extremely toxic! When I used it, we had a special hazmat crew do the hardening step and all of my coating sheets had to be stamped "TOXIC" at that point of addition. Hardening was so fast, that I could process multilayer color coatings at up to 120F within 1 hour of the coating being made.