Something seems to be wrong with a sensor on the rear window.
There is no sensor as such in any window! The motor has current monitoring to detect the maximum up to stop stall and minimum down to fully open stall service position. The window drop is done by counting motor dc impulses back from from the fully up stall position and the same for normal window low (not service position).
I suspect on its individual switch, the electrical current monitored is slightly lower, so its sensing the 'hit'. And on the individual switch, it doesn't sense the 'hit' but because something's loose, its able to physically push its way past the other window and continue.
No, the controller always only monitors motor stall current and the switches just send the logic 'Go' signal to the controller.
...then it goes down, towards the end of its travel it rotates forwards slightly
It may look that way but it isn't. The glass regulator wire pulls the glass up and down in a vertical plane just like the front, but the rear glasses are curved, giving the impression that it rotates. To understand correctly how this all works you need to have removed and re-installed a rear window regulator assembly.
But, if I use the switch that operates all four windows simultaneously, the rear window touches the front window on the way up and goes part way back down.
That is happening because the glasses should not touch, the motor stall current goes up and the system thinks a kiddy has stuck their finger in so pinch protection operates and the system backs the glass down to stop fingers getting chopped off.
Is there a simple way to adjust this so it works properly again?
I bet its something really simple!!!!
That depends on the circumstances? If the front and rear windows were working normally before then something has changed and it probably won't be a shift in adjustment. There is a detailed factory procedure to set gaps between the front and rear glasses so each glass can move against the seal (between the two) be perfectly aligned and with the V.W specified gap. You don't know if the glass misalgnment is caused by the front and its mechanism, or the rear but you should get an idea from how each aligns with their seals and compare with the other side.
Search here, there is a detailed post from me about removing and replacing a rear window together with the alignment checks needed for the front and rear glasses. You cannot do one on its own without checking the other. But you shouldn't mess with either untill you find out why it's changed.
All your problems seem to stem from friction and contact between the glasses causing the motor to protect itself by stalling, stopping or reversing due to pinch protection.