no, it won't void the warranty. they would have to prove the increased viscosity caused the head gasket to fail, which isn't possible.
no point. they can see if you've been tuned previously, even if you flash it back to stock.
clutch can last 200k+ miles if you don't slip it too much at high torque. hell, I've heard of people going over 300-400k miles on original clutches. it's very driver dependent. If you are careful to only slip the clutch at low loads, it will last a long time.
no, not worth the risk.
no one sells a turbo with this mod already done? wouldn't that make the most sense?
i imagine there'd be a big market for aftermarket turbos that sound cooler
a full length skidplate/splashguard that covers the entire front and rear sections. there are aftermarket skid plates, but they only cover the front/engine area.
you can get aftermarket shift cable bushings which will help a bit. but there will still be slop in the shifter. Acuity also sells an aftermarket shifter
mine makes noises too. you can try replacing the spring mechanism in the back. i plan on doing it to mine eventually. I think the springs can bend a bit, lose tension, and make noise. also, the indicate lubrication points on the springs in the service manual.
i do the opposite. 1st on a downhill, reverse on a uphill. probably makes zero difference but the reasoning I've heard is that you don't want the engine to spin backwards if it does roll. but i don't see that happening with our cars. not heavy enough. i think the tires would just slide...