Hey Eredin,
Have you attempted to apply a two sided material to your landscape material and landscape hole material slots?
I have the sun shinning directly from the underside of my landscape and am not receiving any light bleeding. Mind you I am using Cascaded Shadow Maps and a fully dynamic directional light. I have turned off precomputed lighting so my scene is fully dynamic.
Landscape Lit from Underneath
As for getting the sun to turn off at the correct time based on the players position in your level. That is going to take a more intuitive approach and might even require some light coding to work correctly.
If you would like to send me a test project to help you with your issue then take your Config, Content, and UProject files and zip them into a folder. Then you can upload them to dropbox and provide me with the link here.
Regards,