As for what's causing this voltage drop...
car, i have daytime running lights. These lights are off when my parking brake is engaged, but switch back on when i release it. If your car has a similar setup, maybe your headlights turning on is what's causing the voltage drop. That still isn't what i would call normal behaviour for your car's electrical system (unless you have 300W bulbs installed
), but that might
explain what's going on in (3) although possibly not (4). If you have daytime running lights, see if they're switching on/off when you are playing with the parking brake.