Q: Can a waterproof zipper be shortened or repaired?
It can be shortened, but the waterproof barrier is compromised at the cut. Coil and molded resin chains can be trimmed and given a new top stop, and the tape end can be re-sealed with a compatible PU adhesive or a heat-sealed patch. The result is serviceable for light use, but the repaired end will usually fail a hydrostatic test and should be positioned away from the most exposed area.
Metal waterproof chains are harder, because cutting leaves exposed teeth that must be removed and the end re-finished. Replacing a slider is more practical: a sealed slider can sometimes be swapped with the correct tool, but the seal is rarely as good as the original. On laminated tape, heat used during repair can delaminate the film, so keep irons and presses away from the repaired zone.
For a warranty or high-value garment, return the item to the maker rather than repairing locally. If a repair is unavoidable, use a PU seam-sealant liquid on the new end, cure it fully, and re-test with a water spray before the garment goes back into service. Document the repair so the after-sales decision is traceable.