Yes. You can individually configure any button to do any thing.
Have you actually seen any pics of a installed satellight shifter installed on a TT extensions? I am sure it is doable and I will figure it out but looking for examples.
No. I just saw the option to reconfigure the buttons when I was doing my setup. It is on my someday list to experiment with that, but no first-hand experience yet. I have seen the 3rd buttons on the DA road bike brifters control Garmin screens, and that is what is driving my lust for the feature.
Thing I love about Di2 is the flexibility and customization. You can literally take an type of momentary switch, solder the leads to the circuit board and voila your own setup.
Although doable, it would be a somewhat invasive surgery to crack open a shift button, expose the point where the internal button conductors attached to the circuit board, solder a pair of wires in parallel, and then route those wires out of the assembly while maintaining the water tight integrity of the control unit.
I’ve got DA 9150 Di2 on a road bike and Ultegra 8070 Di2 on my gravel/cx bike. To be honest I can’t tell a difference between the two. I’ve got the mid-cage Ultegra Di2 RD that has the clutch, so if I could perceive a shifting difference it would likely be because of that.