Your 'head koi guy' scares me.
No knowledgeable responsible professional in the decorative pond business is going advise adding anything to your pond without seeing the results of specific tests of water quality parameters. A quart of Clorox will get the fish moving around also. Rock salt added in the right proportions will render NitrIte less toxic, but you need to know the level of NitrIte and the capacity of the pond to determine the correct amount to add. Salt has no effect on Oxygen levels.
Koi can handle Oxygen levels as low as 5 mg/L for an extended period of time. I seriously doubt that the level in your pond is that low.
For $150.00, you should be getting a minimum 12"-14" fish. 6" - 8" Koi retail for $25 - $30 down here.
On the Hydrogen Peroxide, it is the extra Oxygen atom that gives it the caustic properties different from water, much like the same extra Oxygen atom conversely changes Carbon Monoxide from a very deadly gas to Carbon Dioxide that plants respire and is usually harmless.
You can retest the pH, if you are confident that it is giving accurate results, but it is normal for pH to undergo some fluctuation daily.
What was the Ammonia level when you tested before? Did you test for NitrIte? If not, test for that also.
Any corrective measures undertaken as regards water quality parameters, whether biological or chemical, should be administered based on reliable test results, not on "Looks like", "Might be" or "It's probably".
"Everybody is ignorant, only on different subjects." - Will Rogers