Environmental control system does not work properly unless connected via USB

Okay, thank you very much for you input, I just want to clarify a few things so I do not make a huge mistake somewhere.

  1. Get the 12v-5v buck converter and solder the wires from the 12v 800ma power supply to Vin and Ground-out on the converter? Would it be better to use a breadboard barrel connector so I'm not cutting up wires and potentially mixing the positive and negative side wires from the external 12v power source?

  2. Use Vout (5v) from the converter to power everything, including the Arduino. Is Arduino UNO's Vin pin the same as VCC?

  3. When grounding everything, can I utilize the ground pin on the Arduino? Since technically, the UNO will only have the 5v connected to the Vin pin and no "external ground" from anywhere else.

The 12v-5v converter I'm looking at is something like this.

Thank you all. You have been very helpful.