RE: JR R921 RX Bind
Once its bound its bound.
The only place you can bind is with the bind plug inserted into the bind port, or with a bind plug attached to a 3 wire wiring harness to the bind port. Spektrum/JR switches have 3 wires, not all switches do. Same with any down stream battery share devices.
But like I said, you can stick a bind plug into the bind port, a 4 cell battery into any servo port, bind and you're done.