I would go with springs with a higher spring rate, due to the extra weight of the blower (it's more than just about final ride height; its about the springs ability to control the extra weight and reduce roll, nose dive, etc.). I'm using bmr sp083 with a whipple. Front settled about 1/4" or so...
Agree with zip ties. I did it with the connectors facing down so water doesn't pool in them. You can throw some tape over them for added measure.
The bumper isn't going to affect your warranty unless something you did with the bumper impacted another component due to a bad install or something...
The steering wheel doesn't really have an impact on the airbag. As long as you reconnect everything properly, it is its own little module/unit that essentially floats there.
I replaced mine last year and used blue loctite on the screw and its still tight and solid. Everything fit together...
There is a rubber boot that goes around the shifter and seals the hole in the sheet metal of the car. When I was installing my exhaust I noticed mine was not fully seated properly from underneath. I seated it and the heat in my console immediately went away. Guessing that's your problem.