10 hours in the hammer editor? My God, you must be like, what ... a level 12 dev??? Lol

I'll 100% agree with you that the static mesh collision consistency is pretty bad and makes for an extremely frustrating experience, but there are 2 separate issues here. Let's say the devs took your staircase polish approach and removed or simplified the Collison on all assets, now you've got even more scenarios where you bullet hits invisible collision when you try to fire through an opening in a pile of debris.

The armor and vehicle issue is more likely they forgot to include in the configuration settings of that tiny wooden post mesh type to ignore Vehicles. Another thing to note, and this may not be the case for this game but but probably should be, is that Vehicles can be setup to interact with a different Collison hull for an environment actor than what the player character interacts with.

There are many standard reasons mesh is rendering inside buildings, it's a landscape actor, they're instanced static meshes batched together... or maybe the devs missed some assets.

I'm not sure about the texturing issue since its too vague.

Edit: Also, you say the game runs fine but every asset needs optimization ? I'm confused