Indeed. I hate all-red signals with a passion, because there are ambiguous states that take longer to interpret than the unambiguous use of distinct colors for braking and signaling. The case you cited (fog) is a counter-example of ambiguity, because you couldn't tell the front from the back of the car, but in most of the cases where a car would have its hazard flashers on, the orientation of the car is relatively less important than its simple presence. Just my opinion, of course.
That depends totally on where the brake/flash logic resides. There might be separate brake/signal wires to the assembly, and the assembly might light up appropriately depending upon the state of those signals, or there may be logic elsewhere in the car. But, Tesla seems to do a lot of logic in the car's computers, so you may be right that a simple parts swap might not be all it takes.
Too bad I didn't have the opportunity to talk to the driver of the car I saw.
FWIW, by the way, I saw another car on the road the other day (I forget the manufacturer), and it had similar inner/outer tail lights with amber signals. My sense was that the assembly was not significantly different in size than the Model S's, and the car was a current model. So, that car was evidently able to meet DOT regulations, whatever they are.
