Damper internals - no change from previous regulation as far as I know.

What could have happened with the mirrors (I don't know for certain) is that the FIA said not allowed, end of story. Simply put, a mirror is a device to give rearward visibility, and technically is not allowed to be or do anything else. I think it's a very different approach compared to F1 (definitely) and other series (probably), and we can push the rules a bit but not too much.

TM could be talking about the side impact regulation. As you may have noticed, most team's first test car was without the extra protection. The problem is, by adding some ~100mm to the door, you need to redesign pretty much the whole aero package. To start, the rear fender/door definitely has to change, then you must consider its effect on the rear wing and try to balance that with new front aero. Or it could be the rear wing, you can see the original version on their first test car (shall we say, "Ford style") versus the current one.