I’m not familiar with Packed Level Actors (PLA’s) but from my quick research it seems it’s just a way of telling the engine “Force these meshes to be 100% static”.
Is it your experience that decals don’t work on these? DecalActors are just an actor that can be placed in space, containing a projection box. Whatever intersects with that projection box will have the decal applied to it (Kind of like you dropped a 2D image straight down through the projection box)
To my knowledge this should stop you from putting a decal actor in the world space of a PLA, but if you’ve already tried that than I can only believe you.
For the screen space culling, you can take a look at:
https://docs.unrealengine.com/5.1/en-US/API/Runtime/Engine/Components/UDecalComponent/SetFadeScreenSize/