Try Paint.net which is a free download and (from my initial view) seems to have many of the functions of Photoshop Elements. I have the latter but rarely use more than a small proportion of its functionality.
I see no reason why you shouldn't swap over the leads for either side at the control box. I have a copy of the installation manual which states precisely this. PDF of the relevant page attached. Reich-Caravan-Mover-manual.pdf
You could still find the caravan brakes overheating as they are applied when the van tries to "overtake" the car. This can happen when the car is slowed down either by the brakes or by being in a low gear as there is no direct connection between the two braking systems.