Cut/Remove Shape out of open Brep or Mesh?

[ilonayo (Ilona)]

I’m new to Grasshopper but having a lot of trouble cutting a shape out of an actual mesh. This shape was created by projecting lines onto this same mesh, so I’m surprised why it didn’t work. I need to cut this piece out to create a similar design to the building shown below. I’ve tried TrimMesh but it’s not working, is there a way to cut this shape out? The problem is ungrouped on the far right hand side.
File below.

Coursework City Hall YP.gh (479.9 KB)

[lars.renklint]

The problem is that one of those surfaces are missing, one is separate from the others (and doesn’t seem to belong to the others), some overlap and some are not adjacent. So it’s not possible to join them.