Chosen Solution
You appear to have done the basics which is about all I recommend. As you encountered the environment you work in also effects your efforts (fighting dust and pollen contamination) I really think you need to step back and re-evaluate if the amount of stain is enough to risk the possibility of damaging the screen. The staining could be dust caught on the back side of the panel its also possible you have heat damage to the liquid crystal and some staining (discoloration) at the bottom edge maybe in fact burn’t reflectors in the backlight LED’s.
So you’ll need to gage your skills and if you have a clean enough area to pull the display panel out and do it without damaging it.
It’s heat related; switch off the screen overnight and the stain is pretty much gone. Let the screen run for a few hours and the stain is back. You can mask it by setting the desktop to black. Improving the internal fan capacity to vent more heat will reduce the incidence.
