ok so the conclusion on the 1615 based on your tests must be that it will only work with a Beovision with datalink I guess.
Yes I suppose that both the BL1615’s AAL socket and the MCL2AV’s AUX/TV socket are expecting to communicate directly with a TV, hence cannot pass commands to each other.
It’s a shame that the stand-alone BL1615 can’t be used to extract the audio from masterlink. Looking at the circuit diagram, there seems to be a couple of points where the ML ON/OFF could be by-passed, but I think that the TV would still need some sort of data command to turn its ML output on (even when the TV is already turned on).