12V External power not fed immediately to Arduino Mega 2560 in 3 board stack

From bottom to top, my setup is the Arduino Mega 2560, Trinamic TOS-100, and a mega-shaped Experiment board with a bunch of components.

There's another board (Temp Board) separate from this conglomerate that requires 12V (11.69 to be ~exact). I'm using 18 gauge wire for the power to those those components. I am redesigning that board so that my experiment board can house the temp board's components and the other components it needs to perform other Arduino-based operations.

So, does the busing system on the Arduino allow me to provide 12V from the experiment board (top), to the rest of the board (wherein the Mega on the bottom is the "last" to receive power)? Or, no matter what, the Mega needs to be the initial supplier (from the power supply)?

To further my intentions, can I provide the 12V to the top, and use a regulator for the 5V for the Arduino operations, and still use 12V to the other components that need it?

I have never done something like this before so if you think it necessary to provide theory behind any incorrect assumptions on my end please feel free.

Thanks
-Anthony