Have you ever used anything in your main tank that had been in the QT with the copper? Fish nets/tongs/water testing vials/rock/sand/cleaning pads etc?
I would say that I am personally attempting to run as 'chemical free' as possible. I did use copper and PraziPro in my QT, and I am dosing Kalk and baking soda in my DT, but so far I have avoided things like Chemiclean and Vibrant. I do have some cyano but I have left it a couple of weeks (other than increased cleaning/filter pad and carbon changes) and it looks, fingers crossed, to be getting better. I have no idea if I would be better off using bottle remedies but I am trying to keep my tank as stable (and natural) as possible.
From my reading it seems that stability is more important than other things. If I understand the salinity reading correctly it really doesn't look that low to me and I wouldn't panic to change anything. I might look at different calibration methods for my refractometer but I don't think you need to do anything immediately. I might put a bit of Cuprisorb in as that won't hurt.
I am not an expert but looking at your test results, it looks like your RODI water is ok, so those elevated metal levels must be coming from either your salt, something in your tank (or mixing system), something you are dosing, or contamination from somewhere/something else.
I also wouldn't shock the QT with the clean you propose if it was my tank. I would do a physical clean (with a brush and cleaning pad) and maybe a small water change. I would then change the filter pad/clean the sock. I would do that a couple of times until it is a bit cleaner. I then might vacuum little bits of the sand at a time with a couple of days break in between (with frequent filter cleans/changes and occasional small water changes if necessary). If you're still not happy with that in a couple of weeks then maybe you could consider either the Vibrant or a coral dip (if it really needs it). If after all of that you're still not happy then consider what next. If it's as bad as you suggest and you do that all in one go then you risk upsetting the balance again IMO.