I recommend keeping this project at the modular level as follows:

1) Power module (I use 12v in, 5v out)

2) CPU module

3) Button module

4) Relay module(s)

For the main lamps, I use the 12v rail wired via a relay so that the lamp receives the full 12v whilst ON. This can be (and probably should be) tested independently from the CPU module.

The button line is pulled high via the 10K resistor and awaits a momentary push to GND to activate.

The CPU module is last to wire up, once the Power + Relay + Lamps have been tested separately. As the button is directly coupled to the CPU, it has to be tested together once it is all wired up.