About time!
Did I not explain that back in #4?
And it is not at all a "12v jack". If you must, a "7.5V jack".
It's a "legacy" part. Once provided, it is provided in successive generations "just in case" - for "compatibility". More recent designs actually do have proper regulators.
What gives me the shivers about your last diagram is the implication that the data lines to the LED strips (and the 7-segment displays) are actually separate from the grounds. 