Mysticblue, you asked enough questions there to start a whole new thread =) I won't answer too much because it is offtopic, but most of your questions are general questions regarding turbos in general, and you should do a fair bit of reading if you want to get to know your engine better.
That link has a schematic of the N54 with regards to the turbo, and 15 is the chargepipe, and 9/13 are the diverter valves, 7/14 are the pipes that carry air from the diverter valves to the beginning of the cycle and 10 is the intake.
In short, the chargepipe is the pipe with pressurized air after it is compressed by the turbo and cooled by the intercooler.
The diverter valves are connected to the chargepipe and to the intake, and are used to divert compressed air when you let off your gas or change gears, as this creates a sudden drop in air pressure and without the diverter valves could mean that the pressurized air would rush the other way (back to the turbos) and potentially even cause damage. Instead, the air is thrown back into the intake and makes it way around the whole system again.
The Forge diverter valves have changeable springs, which means you can adjust the pressure at which air would be recirculated. The stock ones are set at a given level and at some point could create problems holding boost if you are going well over stock boost levels.
Blow-off valve vs. diverter valve is a long topic without clear-cut answers in my opinion. With bigger turbos diverter valves are good because pressurized air is entering the intake instead of normal air, which means after changing gear/letting off the gas you start at a higher boost level rather than getting fresh air and starting from 0. Our N54s most certainly are not big turbos though. On the other hand, the air in the chargepipe is hotter than what the intake is taking, so if you have diverter valves you are recirculating hotter air, which is detrimental to performance. Then there is also the matter of preference regarding sound - blow-off valves give that famous ppssshhh sound whereas diverter valves are much quieter. Basically you're good with either on our platform, it is not a big deal.