Hi, thanks for your comment!
I'm afraid the answer to this is plain old human error. I messed up this level and it's a lot easier than it's supposed to be. The little corridor at the right linking the upper and lower parts was a late addition, and for some reason I didn't notice that this made most of the challenges in the level pointless. Don't know if I was tired or not paying attention or what, but yeah, my bad.
I will fix the level when I get the chance.