which DAC is used when Audioengine is plugged into Rega DAC

If you plug it into the Rega DAC you are using the DAC in the Rega. Better cables do make a difference yes. I can't answer your other question perhaps others can.

Your question is not 100% clear but it sounds like it is related to Bluetooth, and which bt receiver would sound best?

As long as you have analog rca cables coming off the rega dac and a digital signal going in, than the conversation from digital to analog is being done by the rega. In a setup like this, there is zero chance you will hear the difference between optical cables that are functioning properly, but that is not to say that buying a decent cable is not a good investment as it will likely last a lot longer. 

If this is about Bluetooth, you want to understand what protocols are supported on your devices. Ideally you want aptx hd, but both you phone and the receiver need to support it.