Thanks for lengthy reply 
I already have mesh terrain working without any crazy analytical tricks. It’s 100 km^2 (10 km x 10 km) with complex collisions. It works great, except there are seams between terrain tiles: #124475 - Custom vertex normals do not get exported into FBX correctly - blender - Blender Projects Not sure if it’s Blender’s issue or UE issue. That’s why I decided to try Landscape.
In fact, I finally got Landscape of the same size working even better than meshes. Looks lesser detailed, but still good enough. Performance is better than with meshes too.
In the Editor, collisions look fine. However, on the device, they are wrong for some reason 
LOD0 in Editor:
In-game (on the device):
As you can see, yellow reticle either hovers over visual landscape or sinks in it, even though collisions match visual in the Editor.