Hi,
It is possible to test all the glow plugs at the same time without disconnecting, but you require a current probe, the one that has openning jaws, and a DVM.
I have just checked mine and from cold they take 80/90Amps, dropping to about 50/60Amps at the end of ther timed period.
The jaws of the probe that I have are too fat to allow a test between the plugs.
Regarding the 8mm nuts, I use manetic tip from a screw driver, holding the tip on the middle of the nut with one hand and rotating the nut with the other, untill the nut bites on the thread.