I am looking for help with a solid state unit that will allow my 12v DC motorbike lights to work after 10 volts are reached from its alternator. Thanks...

I've got a junction box in my attic that is fed by 12/3 cable. Of the 6 cables in pic below, one is the feeder 12/3. 3 are switch legs (1 running to light switch, 2 running to 2 different lights controlled by the switch - not sure if switch leg is the proper term for these 2 cables). Setting those aside, there are two remaining 2 conductor cables being fed by the 12/3 cable (both exit the j-box from the top). A 14/2 cable uses the black hot from the 12/3 feeder, and a 12/2 cable uses the red hot. These two cables power bathroom heater/fan/light combos and a bathroom outlet. I haven't crawled far enough toward that end of attic to confirm, but as far as I can see, it looks like one cable runs to one bathroom and the other runs to the second bathroom (I can separate each of these two cables at the j-box and confirm there is no interaction at some device downstream). 1. Why bother with 12/3 cable to feed this junction box? This originates at main panel as THHN 3 conductors in flexible metal conduit that runs outside the house and up to outside of attic. It enters via an external light fixture's junction box where is connects to the 12/3 that feeds j-box in attic. So at 2 points, the decision was made to use 3 conductors (3 conductor THHN, then 12/3) instead of 2 x 2 conductors (4 conductor THHN, then 2 x 12/2). Seems to me you could have exact same functionality using 2 x 12/2 and you'd have 2 separate breakers instead of handle-tied breakers. 2. I'm planning on adding recessed LED lights to a room (attic is above this room), so I was thinking of tapping into this j-box for power. I guess it doesn't matter which hot (black or red) I tap into? 3. Is stacking junction boxes to get a deep box code compliant (in an attic, not a living space)? Looks like the j-box has a rear panel that can be removed. Pretty cool. If this is code complaint, can I add another one? I think there's enough room in this stacked double box to add my 12/2 for the LED lights, but would be good to know if I can add another box. 4. If you have 5 wires to twist together, do you twist 3 together, 2 together and twist those two bunches together? Or pigtail each bunch and twist pigtails together? 5. What section of code should I browse to get a handle on attic wiring? The oldest wiring is an absolute mess with cables just draped over joists. The stuff done later was much better with cables stapled to trusses and at much lower risk of being trampled.