First of all, Lightmass does work well with Landscapes! The bigger problem for Lightmass is looooots of vegetation. Thats where you have to work smart to make it work.
A couple of things to think about:
- I would always use the skylight and directional light in stationary mode since static doesnt give nice results!
- I would always use stationary local lights as well until you start adding fill lights…those would be set to static instead of stationary then (you dont get specular from baked static lights)
- Also, your shadows probably disappear because the landscape lightmap rez is not high enough to make shadows if you use static lights. Using stationary lights should fix this
- Shadows reappear when you move the light because its showing you a fully dynamic (not baked) shadow as a preview
- you can also have a bigger landscape, and then move some components where the gameplay area is to different streaming levels and then bake the game area in higher quality than the rest of the landscape making it easier to work with Lightmass and its baking times. There might potentially be seams where the 2 parts meet, but you could hide those by placing objects there
I would highly suggest that you check the link in my signature since I just uploaded a video about lighmass in an outdoor environment and how to set things up so this might actually be the perfect help for you.