2) I would like to learn about a 12V source, switched with the ignition (i.e., "hot" only when the bike's running), to power some LEDs I'd like to add up front, and maybe in the rear, too. I suspect that there's such an unused connector inside the headlight nacelle, but I'm not sure.
Yes, same as the one one that feeds the USB-A, inside the light nacelle next to the USB one.
3) I believe the connector for the Tripper -- which I don't have on my Meteor 350 -- is also in there, and might do the trick for me (with the proper connector or DIY splicing .
Yes, tiny little 6 pin one on mine, only bottom 3 pins used, good luck finding the type of plug, definitely a DIY splicing, if you come up with the plug specs, please tell everyone, you would get a few free coffees.
4) It wouldn't shock me (get it?) if there's also such an unused connector with all the fuses and suchlike, behind the right side panel, although I don't recall reading anything along those lines.
A Euro 4 and Euro 5 connector exists on my bike behind that panel, I used the Euro 4 one to power the Daytona Velona 8K Tachometer that I fitted, sourced a compatible plug to do it. Can unplug it to connect my OB!! scanner if needed.
You would have to peruse the wiring diagram to ascertain the fusing/ current capacity of those options.