The composite exercise (installation part) of AM2 is a bit like the driving test, there are marks deducted for minor faults but there are also certain "critical points" which are instant failure (sorry, referal, nobody "fails" anymore...). Anything to do with earthing & bonding, tightness of conductors in terminals, couplers on conduit etc. is a "critical point", get these wrong and that's it. Safe isolation & other health & safety parts will also be critical.
From the feedback I get from my students (& we've never had anyone referred yet) you can have all conductors on the MI short out & still pass, as long as the conductors are correctly identified at both ends.
The overall qualification is split into sections,
A1 Composite installation
A2 Visual inspection
A3 Functional operation
B Inspection and testing of the composite installation
c1 Safe isolation of test unit (including risk assessment)
C2 Safe isolation of supplies
D1 Fault diagnosis
D2 Fault rectification
My understanding is if you are referred on any section you only have to retake that section, not the whole thing.
Hope this helps.