The third tab is just for connecting the diode too. If you were to disconnect the red wire and the diode from the left tab the lamp will still work. But in doing so the red wire connected to the diode will need to be insulated. Also the diode lead may break over time due to flexing and vibration because it isn't attached to something solid.

The voltage will still be the same, the extra current not being used is not a problem. The lower current being used will lengthen the life of the power supply and connectors.

Yes that will work. I would use heat shrink tubing to insulate from the wire all the way to the socket as it will help support it. Also maybe two layers with help support more. Then tie it to the yellow wire to help support it.

Make sure all mounting screws are installed and tight. Keep wires away from the display. And you may need to recap the power supply.

I wasn't sure if you changed the power supply and or the MPU board with a Rottendog. There is also the big blue cap in the back box that can give trouble sometimes.

Leds can be over powering, must wear shades