D7 Rules - Unable to get data value & Unable to evaluate action in custom module
For any future readers who encounter this issue:

It turns out the answer all along was using the hook_commerce_checkout_complete in my module, rather than trying to insert an action with rules.

Here is an example I used for debugging:

function MYMODULE_commerce_checkout_complete($order){
  $file = fopen("checkout_debug.json", "w") or die("Unable to open file!");
  fwrite($file, json_encode($order, JSON_PRETTY_PRINT));
}


of course the reason I used json_encode is because $order is not a string and I could pretty print it.

Hope it helps someone, eventually. Thanks again.