are you sure they are backlight issues? there are horrible copy aftermarket screens getting around that do the same thing work fine then screen goes black with only backlight shining. combination of home and power button presses sometimes work but more often then not you pop the battery the power cycle phone and it will come on again fine and work until it fails again. id be leaning more towards a screen not a backlight chip
sounds like the cable on the screen is faulty and you may have possibly damaged the original screen cable when you removed the button those buttons are very fragile and open to easily getting damaged on removal if your new screens come and still same issue put original screen on a send them back to apple with a refund
when will people learn to pop the battery before changing a screen! the 6s series up still carry voltage even after being powered off so it is essential you pop the battery to stop blowing the backlight chips. with is exactly what has happened to the poster. no matter how many screens you try they will all have the greyed out part that will continue to get bigger.
have you tried power cycling the phone? pop the battery and press the power button then plug back in. if that doesn't work you could have blown the backlight chip by changing screen without disconnecting the battery first. rip the backlight off a working screen and shine a torch behind the lcd to confirm