Well, in your set of tests, you reported that you had dust coming off when you brushed at one point AFTER you'd been all clear, so maybe not. eg, you've tested for metals, the sun makes it disappear when there's no water, and it is to some extent affected by direct chlorine. This leads me to assume that it is organic, and that it is likely mustard algae residue that then depopulated.
Check this thread and see if you followed those recommendations or maybe bring it up to mustard shock for another 24 hours --
pool-school/mustard_algae
Did you, for example, clean out light niche, disinfect all the equipment?
And are you actually staying above your shock level at all times, or are you letting it drop below before bringing it back up? That could be making a difference. To me you post sounds like you're dosing it, then checking at night. If instead you went above mustard shock level and then had someone at lunch bring it back up, etc., you might see better results.
I'm sorry if I've missed something you posted elsewhere, I'm not certain I have a full picture, but by the sounds of it in the past you let the chlorine drop and the stains reemerge, which suggests they're organic...and then they get worse. So it sounds like you're still dealing with active mustard algae, which causes me to ask whether or not you followed the advice in the thread and treated every nook and cranny or not. Lots of folks go on for days or weeks shocking, only to find algae hiding behind the light niche for example.
If you've exhausted all those possibilities and given it another good 24 hours at mustard shock level, you may wish to try another shock session but possibly boost it with polyquat 60 -- not cheap and not bbb, but some people have reported success with it (except it will increase your chlorine usage). Other people use enzyme treatments, as enzymes eat algae. Can't speak to whether it is effective. Thorough, consistent high shocking is what *should* work. Sorry I can't offer more assistance.