The 4.11 preview now has lighting channels which might be worth looking at, it states in the tool tip “These channels only apply to opaque materials, direct lighting, and dynamic lighting and shadowing.”. I have no idea if these channels work with mobile however.
Reading up on mobile lighting here https://docs.unrealengine.com/latest/INT/Platforms/Mobile/Lighting/index it seems the modulated shadowing that you are getting is intended and at the moment there isn’t a way around it without it being too expensive to use on mobile platforms. If you check out Dawn Of Titans game play, which is a pretty big budget mobile game, their shadowing does the same thing. Real time shadows on mobile is a mine field, I worked on a project in Unity recently which required real time shadows, the results were never pretty and we could only target really high end devices. If it were me I would move the buildings and angle the light so that the baked shadows and modulated shadows rarely overlap, I would also consider having blob shadows for the turrets on lower spec hardware.