Another two cents:

As previously mentioned, adding the hardener directly to the gelatin is much more efficient, eliminates a step in the process, and does away with having to fill a tray with malodorous aldehyde. IIRC, sodium or potassium metabisulfite (also call pyrosulfite) will neutralize aldehydes if disposal is an issue. Glyoxal can cause yellowing if used to excess, or with papers that contain a starch sizing. Rinsing afterwards will usually lessen the problem, though I would change hardeners if the yellowing becomes an issue.

My personal preference for hardening is glutaraldehyde - very potent and I've not encountered any yellowing issues.