When using a bicycle rack on the factory hitch and a dummy (tester) trailer electrics plug to kill the PDC, I assume it will also activate the Trailer Stability Assist. In doing so, what if anything happens to performance, gas mileage or handling since there is in fact no trailer attached?

There is no shutoff on this LR2 for PDC hence the dummy load.

Also, are the trailer electrics turned off when the engine is off or does it stay live (battery drain)? Wanting to leave the tester plugged in as the bicycle rack will be on the LR2 most of the time. 2014 LR2 HSE LUX in Fuji White

Trailer Stability Assist requires no trailer to be present for it to work, the 'yaw sensor' is always active and the individual control of each wheel will happen solo if car is unstable or if instability to the car is caused by a trailer.

Not sure on your trailer socket but the UK 13 pin has a permanent live on one pin whether ignition is on or off.

Thanks for the detailed explanation. 2014 LR2 HSE LUX in Fuji White