When it comes to the wiring error, it seems unlikley since the backpack pins are connected to the right spots into the display from 1 to 16.
I might try to desolder the backpack from the display and see if it works with normal wires.
If if the display had power, the backlight would work, it would show black squares instead of characters, ...
... and the green and blue backlight on pin 17 and 18 would work, since they're grounded trough the backpack.
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via Egeo 16