re the extra layer of protection - you might be adding in errors / compile times and effecting game play - see this for details:
https://tf2maps.net/threads/guide-hl2-map-editing-optimization-page.37987/
After doing a compile, copy it and paste it here - it will give you some possible solutions:
https://www.interlopers.net/errors/
Leak line goes straight through a wall? Check these out:
1. ctrl+shif+w on the brush the line goes through - run compile seek if you get the leak same place
2. recreate the brush if needed, it might be a bad brush
3. look for models near the map edge, their bounding box might be on the outside of the map. How do you find the bounding box? select model, go collisions, use drop down boundary box - it will show you the actual size of the prop - if the yellow line is in the void (outside of your map) you will get a leak.
4. it might be something else causing the leak and the engine doesn't know how to tell you so it draws a line
a. delete/hide brush - run compile, does it leak same spot? if no, fix that other leak first then redo, if yes..
b. cordon tools method - use it for all other parts of the map - seal any leaks there first and come back to it
or
c. big box method - put a big box over the area - fix any other leaks and keep working bag to it.
Have fun!