Last night I tried the "Y arm + central part" locking again. Three different configuration were investigated :

•  Y arm + DRMI
•  Y arm + PRMI
•  Y arm + MICH

In all the configurations I displaced the Y arm by 20 nm from the resonance.

As for the DRMI and PRMI configurations I wasn't able to acquire the locks.

As for the MICH configuration, the MICH could be locked with AS55. But after bringing the Y arm to the resonance point the lock of MICH was destroyed.