1) Hot-melt glue - its ugly AF but works. Most builders use 3 wire RC cables with a single connector at each end.

2) You can usually just solder up a Y cable but a more elegant solution is to slave two channels together in the transmitter's programming and use separate cables on the receiver.

3) Why do you need two spring loaded sticks? The Rage has built-in channel mixing so you can use just one stick without any transmitter programming for steering and throttle. You can also look at a 'wheel' radio for RC cars, which has a trigger for the throttle channel and a sprung-loaded wheel for steering