Here is an updated diagram showing the Rx power.



Note that there is no voltage regulator between the Rx battery and the Rx.

Here are the wiring changes I made:

ELEVATOR
- shortened both servo leads
- replaced connecter between Y-harness and extension with a solder joint

RUDDER
- Shortened servo lead: wires cut and new connector installed

IGNITION
- shortened ignition LED lead: wires cut and new connector installed

Note that no changes were made to the throttle wiring (except to disconnect it from the receiver, re-route and reconnect). When the original servo stopped working I checked and noticed that its connector into the receiver was reversed. However, with the power connection in the centre of the plug it's my understanding that this can't cause any damage. Could this have damaged the servo or receiver?

When I tried the spare servo in the Rx, it was connected neither to linkages nor extension - I held it in my hand and plugged it directly into channel 3 on the Rx.