Interesting point. I think there’s SOMETIMES a fine line between scrying instrument and evocation base (incense, steam, statue) and it’s like the way there’s a crossover in some cases between divination and evocation.
The agate slice I use in place of a triangle doubles as a mirror and that was kind of the concept behind it, though you’ve elucidated it far better!
Honestly if you want to improve your vision just learn to quiet the mind. You’d be surprised what you see and understand when you still yourself and are receptive with out expectations.
GOTO BED MAN YOUR ON THE EAST COAST TOO! 
Nightshift motherfucker, getting paid double time to play on my phone lol.
Now THAT is the best way to work! ROFL!
actually scrying eyes open is not easier for beginner, on the contrary it’s harder to focus this way, reason why eyelids scrying tends to be more beginner friendly
seeing purple lights and shapes when you scry with closed eyes is not necessarily related to the 3rd eye (I wish it could be). Those things are natural discharge that everyone closing its eye can “see”. However they are a good starting point to scrying or phasing.
In any case, best thing is to try both : scrying eyes opened and closed to see what fits best to you, knowing that scrying is DAMN HARD anyway !