Hi MrsKat - welcome to the forum!
It does sound like you've gone through all the necessary trouble-shooting steps and it is quite perplexing for me too...
Perhaps one last thing to try is to remove the power track from the layout and use just that one piece for testing. Check again that both controllers and both cars work on lane two. And then try them on lane one.
If both cars and both controllers now work on both lanes, then I would guess there is a short circuit on one of the other pieces of track - possibly a small piece of metal or braid stuck in the slot. Fortunately there aren't many track pieces in the Endurance set, so it won't take long to give them a good inspection, hold them up-side-down, use a soft brush (eg a cheap foundation make-up brush) to clean out the slots and give them a good puff. Then build the circuit and try the two lanes again.
If using just the power straight doesn't solve the lane one problem, it's possible there might be a short in the slot on the power straight, which can be cleared in the same way as above - up-side-down, brush, blow... If that doesn't work, I'd say the powerbase is faulty and to contact Scalextric Customer Services (details either to the right or below).