The trigger is one of the PITA parts in a gearbox, consider yourself lucky it's not a V3, those are even harder to keep in place.
The first times you reassemble a gearbox it is normal to have this kind of thing happen, sometimes you get to place the trigger in a manner it won't pop out. Otherwise you need to hold the trigger and the spring with a hand while placing the other half of the shell close enought so they don't pop off.
The spring does go where you point out, according to it's length. You might be able to stuck it between the trigger's electric plate and the shell it you can.
EDIT: I had to pop the image in a new window to view it
