It won't be floating. I run the same cams, springs and retainers and have given my setup 30 psi. Not sure how much more boost you want to pull out of it. It's dropping to 18 psi as is.
Never had an issue. Even down at Cootamunda in the middle of winter, left outside at the hotel. That was bloody cold that morning too. Get the tuner to do a PROPER cold start.
Have used a Jaycar trickle charger on my R33 GTR for the past 8 years. I come in, open the boot, plug it in and walk away. When I want to use the car I'll unplug it and drive off. Never an issue.
Your NISMO clutch will work.
Stock ratio is 0.757, the OS is 0.826
Assuming a 265/35x18 tyre and IF my maths is right, at 120 km/h with a stock ratio you'll be at 3088 RPM. The OS 5th gear at 120 km/h is 3404 RPM.