sounds like a shagged actuator to me. but check all the couplings anyway, it could still be a leak. but the variation in boost would suggest a grumpy actuator.
not me personally, but my car is a wee bit more modified than yours the emanage blue is more than capable of running stuff up to a td04 pretty well. they are good little piggybacks.
run the engine till its up to running temp, then turn it off and pull the plug leads out. if you got any steam or anything coming out, youll have water in the leads. and mate, get a proper boost gauge, 5 o clock? wtf?
check your boost. and does your engine light come on? if youre going over 13 psi then youre hitting the fuel cut... if not, and its just everytime you plant it, youve probably got water on the plugs.
you do have a knock sensor, they are stock units on the car, and seeing as youre piggying, the stock system will still be in place. and you wont need the boost sensor unless you want to run more than 1 bar.
as long as the old bottles have been sealed youll be ok.. if the lid is not tight enough though, it will be ruined. i ALWAYS buy new fluid because as i said before, its hydrostatic and literally sucks the moisture from the air. and it ruins the fluid. and yes, use 5.1
id to it on purpose mate. id just lower the boost to like 5 psi, and rip up there in 3rd. you havnt experienced noise until youve heard my sceramer pipe, it sounds like an jet taking off.