So I watched your video, and you mentioned not understanding visportals properly. You're pretty much right when you say the mission's so small that visually it's not that important. But they also control sound propagation, so even in a tiny mission they're crucially important.
You might have noticed, for example, that if you jump around on your starting ledge, it's easy to alert the people watching the museum exhibits. And when they start barking, it sounds like they're standing right next to you because, without visportals, sound propagates in a direct line. Realistically, it should sound more like the sound has traveled through the open window and round the side of the building- so it should arrive from a different direction and be a lot quieter.
To get this to work in FMH just for the starting ledge, try this:
* Extend the walls of caulk all the way to the ceiling, and make sure there are no gaps. You want to seal off the ledge from the rest of the mission air-tight, so that not even a gnat could fly into the museum except through the window.
* Make a worldspawn brush inside the window, blocking it off entirely. Convert it to a visportal. It should now be a cuboid with five nodraw faces and one visportal face.
* Dmap your level. If you get any "portal dropped as leaky" errors, then either the visportal isn't blocking the entire window or the other brushes surrounding the ledge area aren't providing an air-tight seal. If that happens, load up the pointfile in DR and look for the red line. This indicates how our imaginary gnat is flying to avoid having to pass through the visportal. Extend the visportal or caulk brushes to block the gap, repeating as often as necessary until all the leaks are sealed.
Do all this multiple times, putting visportals into the gaps between rooms. For example, in the coal room, obvious places for visportals would be the hatch leading down to the exit, the locked door, and the hole you jump to from the chandelier. Other natural visportal locations would be doorways, the little crawlspaces, and the vent leading to the room with the Mysterious Key. You definitely want to extend the caulk brushes near the balconies to be completely air-tight, because right now they seal nothing and I can even mantle onto them and onto the museum roof. The goal is to divide the mission into discrete areas- visleafs- where you can't get from one to another without passing through a portal. A noise that happens in a different visleaf will sound to you and to AIs as though it's travelled through the nearest portal and not in a direct line through a wall.
https://wiki.thedarkmod.com/index.php?title=Visportals#Sound_Propagation
LOL, good luck with that brightly lit loot platform with the museum guest watching it constantly from three feet away. That's 94.1% of the mission's loot total. XD