Thought I should post here, since it's a bit more of a tricky one. Missing the final 2 splits on surf_cyanide. I'm not sure where they're located but my guess is for at least one of them I'm avoiding it by doing the skip on stage 2. I've outline the traditional route in red and the skip in blue, I'm assuming one of the zones is at or inbetween the windows in yellow, and possibly if there's one when it opens up to the sky that could also be avoided as the skip is taken fairly wide. Hopefully this helps and zones can be resized/placed accordingly.
So I made some changes earlier (after getting a report) before seeing this forum thread and you're correct in assuming where the yellow box is roughly (extending back to the ramp) where the final split is. I say final because it acts as a double checkpoint ending split 5 and starting split 6 (which is why you miss two splits). I'll look at this in a bit more detail in a bit because I honestly wouldn't have assumed what you were doing is even possible. I think I even tried a few alternate routes and either hit invisible walls or teleports so I assumed this wasn't even possible.
There is a teleport/invis wall which is why you can't directly go towards the ramp, you have to go wide and allow yourself to travel past and fall below the respawn before you can turn around.