Being at home I can't verify power flow but the B2 problem only affects going forward if I recall. I think reverse is just the forward brake discs (clutches that anchor to the case) and the one way clutch. Could be wrong though.
I would check your system pressure. I have seen a number of front pump problems cause a no-go situation.
Do you get a speedo reading when you rev the motor in drive??
Bosch Master, ASE Master, L1
33 years MB technician