Why is screen green and flickering?

This is a repost since I didn’t get a resolution previously.

A couple weeks ago my screen turned green after a few seconds of booting up. Here are pictures of the same image on my broken MBP and a functioning MBP.

Characteristics:

• Some parts of screen flicker - different parts flicker depending on image (see end of album for video of textbox flickering on Wikipedia)
• Screen changes shades of green over time (usual deepens)
• Images appear normal on external monitor (via firebolt/hdmi)
• The screen reverted to normal colors for an entire day yesterday, but is back to green today

Assessment from Apple genius bar appointment:

• It's definitely the motherboard
• The GPU looks like it's toast
• ~$500

Assessment from computer repair shop:

• It appears to be the display
• ~$350

What I've tried:

• Reset NVRAM/PRAM
• Blow dust off motherboard
• Disconnect display cable from motherboard and blow air over connection to remove dust (from this answer: Display Problem : Green on black color parts on screen)

Potential solutions

• Replace display cable (LVDS)
• Replace LCD
• Replace entire display
• Replace motherboard
• Clean off some other component

I suspect Apple was wrong and this is not a motherboard issue because of normal external monitor behavior and the screen sometimes works.

Does anyone have any insights? Any other solutions? What should I try next?