"the closest attacking model must be moved to contact the closest model in the enemy unit against which the assault was declared. Then remaining models can assault models belonging to other enemy units"
Quoted again from the "assaulting multiple enemy units" section. Doesn't this part mean that you don't have to move all your minis towards the unit you directed your assault in the first place? After the first mini is moved, all the ret should be able to assault who they please, as long as they keep unit coherency.
Either way, I REALLY have to go to sleep (5:45 hours:minutes is all I have left), so I'll look into this again once I'm fresh tomorrow. I'm probably just not thinking clearly anymore.
EDIT: I'll also check your diagram tomorrow. I can't even understand my own diagrams right now.