I fixed it by replacing the faulty GPU. Same issue; only video at the external video output. I tried another LCD, inverter and powersupply. No succes, so I replaced it with a K1200M with custom firmware. for this issue it’s crucial to know if the external screen is attached as secondary or primary. logon screen at secondary screen: LCD is not being. Initialized due to loose connection. I found out troubleshooting that the external screen is mounted as primary when I disconnect the internal lcd partially. The external screen get primary if the internal lcd is not connected right. if you get a loginscreen at the external screen, this might indicated a loose connection. with the logon screen on the faulty internal LCD you will get a blurred background visible on the external. In my situation this was a result of a faulty GPU that didn’t feed the internal LCD properly.
I’m having the same issue. MacBook Air, A1466. Powerbutton doesn’t work. Tried the whole package, reset SMC, loosened the battery, reset SMC. Again and again just to power it up after shutting it down. I was already having trouble to shut the Mac down. So a lightbulb started to light-up above my head. The power switch was suspect. The power button didn’t respond, also while running MacOS. I connected an external keyboard with an power button and voila, MacOS asked directly what to do, power down, sleep etc. So I think it’s the power button. Small chance of a defect logic board, bigger chance of a defective keyboard.