I know that a receiver can power on automatically when it receives a command over HDMI (CEC). For example, if I turn on my TV or Apple TV, they will tell the receiver to also power on.

As far as I can tell, I can't get a bluetooth audio receiver (APTx receiver) to get a receiver to turn on via optical cable. Only HDMI has this capability.

The receiver seems to be able to receiver some 12V trigger but not sure how that gets used yet. And my current BT receiver doesn't have a 12V output.

Is it possible to connect to a receiver via a BT receiver (receives audio signal from a phone / tablet)? I think not but not sure. If possible, what device allows it?

If your BT audio receiver can be operated via a remote, then a universal programmable remote might be your easiest-to-accomplish option. I have never heard of multi-device control except through an HDMI connection between units.

I wonder whether there may be some on/off power switch doohickie that would tie into the HDMI and control your BT device. Easy enough to do with an Arduino and relay, someone must have designed/built one, but prob. overkill for your needs.