Considering the limited range of Bluetooth, I wouldn't consider that a viable option either, then you still have audio latency you are adding. The second best solution, IMO, is a unbalanced to balanced audio converter pair. This will help eliminate any noise in the system.
For the Echo, it is better to have a local unit in, by the amplifier, and you can tell one Echo to play something on the other Echo. This keeps your cable runs to a minimum.
For the projector, I would use digital audio the entire distance, then throw a DAC right at the amplifier to ensure low noise in the system.