You can take the light out and use 3m panel bonding adhesive and set it back in place you'll just have to give it time to dry. Way better than epoxy on soft plastic
Drivers side is the harder side to get into. There is a module and bracket in the way from comfortably getting a hand in there. Hopefully the tabs that are on the light housing itself are not broken
Best practices. No scotch locks it's a bad point of failure over time. Pac audio lhfd31 t harness. Then run high level to the sub to make life easier. But since you already have a LOC keep rca.
He has it correct but the rear input to the amp which is what would sum to the rear channels to the sub has a built in crossover on the factory head unit. So if he only feeds from front he will lose the fader option which in my personal opinion is fine.
B&O is good. They have limits on factory so you do not blow the factory speakers. Only way to increase sound is aftermarket. By changing the speakers it might get a fraction louder but the trade off is the sound out of the aftermarket speaker is not tuned for the surroundings and the dsp...