So many questions! Easy Tiger, let's deal with one thing at a time.
So, we have both loco and tender running..
1. Do they run in the same direction?
2. Do they both respond to the same address. - Both move when the same address is selected on your Cab?
3. What is the value of CV 29:
For the loco?
For the tender?
Amongst other things, CV 29 sets the number of speed-step, and whether a 'standard' or 'custom' speed-curve is used. - If these do not match, the two units will not start and run at similar speeds.
Rear Light:
Yes, you can use the LR and Dec + connections, but there will be a CV to set a divider. You use this to match the lighting voltage to the rating of the bulb.
If your rear-light is a plug-in bulb, does it have a flat end? - If so, it is most likely a 5V bulb, set the divider to '6'.
This light will only be energised (on) when the tender is running in reverse.
We can get the speeds right, and the rear light working first, then start looking at the sound..
PhilP.