I think I will just go with one entrance, where the real exit is in the original. In the forward level, you have to go from the entrance to the 7th exit, so it makes sense that the reverse level should make you go from the 7th exit to the entrance.
The leftmost exit working a known bug, IIRC the original level has a gap in the index (z-coordinates) of objects. Lemmix doesn't handle this correctly, it bumps the leftmost exit to z=15, which causes it to have a working trigger. Interestingly, the second exit (z=16) seems to work here too, despite the trigger area not being displayed, I'm pretty sure this is another bug.
In other index-related quirkiness, on Flurry 3 the entrance is at z=18 (thanks to all the decorations
), so I had to change that in order to get a real exit.