Well, I solved some issues, and got this so far. To accomodate for the chapel I pulled out the rock wall a bit (SSS+surface shader lateral displ), and to get the fall/water follow the terrain I switched some SF's places. Two holes burnt in the sky for the local light.
I don't think I'll completely get the painting copied. Might be possible with some careful landsculpting, but it's not worth the time. Quite satisfied with this already.
I'll have a look at some parts again, and maybe improve. Any suggestions are welcome, of course.