The clutch M/C is most likely the internal seal around the piston bleeding pressure. It doesn't leak externally which makes it more difficult to diagnose. Look in the reservoir, see if the fluid is dark toward the bottom. Replace the M/C. I had the same issue on my GSX.-Ron

Open the radiator cap while doing the leak-down test, you may see small bubbles in the coolant. It could also help identify where the leak is.

Coolant ports are M14 x 1.5 metric thread and need to be sealed with a crush washer. See link for adapter parts http://www.atpturbo.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=tp&Category_Code=GTBB

Yes, it is normal for the sensor reference ground and shielding to be tied together. The crimp ring at the junction looks OE.

Do a Google image search. Will better show you what your looking for.

It's a small plunger switch with a 2 pin harness & a 14mm lock nut for adjustment. Look at the pedal, follow up toward top of pedal, there will be a small bracket with a plastic pad on it, the pad is what contacts the switch. The OE p/n: 36750-SE0-013

Check the brake switch under the dash at the top of the brake pedal. They are known to go bad, and the gear shift interlock is not disengaging.

What you described sounds like a severe boost leak or a problem with the MAF harness or MAF itself. Double check all IC couplers and clamps, then the MAF harness & unit.

There is sometimes an arrow cast into the top of the piston, this points toward the timing belt. The larger valve reliefs toward the intake side.