Hot damn, this is nice. Just about the only issue I can see is that the grass cards aren't blending with the ground very well, especially apparent in that last image.
If you need to paint on pieces that are very close together, switch your brush alignment setting to UV, this will paint in UV space and will not bleed over the other pieces of geometry.
You can use masks or vertex blending, both has advantages and disadvantages and there are a lot of tutorials around. This one is the first to come to mind. http://www.hourences.com/book/tutorialstextureblending.htm
The blend map is called a Splat Map. Have you tried searching with this term? I would try variations on "unreal splat export" or "unreal landscape texture export".
Here's a decent list from Mashable about vulnerable sites from the heart bleed attack. http://mashable.com/2014/04/09/heartbleed-bug-websites-affected/
that sculpt is great, nice work! You might want to set up a material blend based on height for that broken concrete texture. https://www.youtube.com/watch?v=dghCetkArJI
they are quite short, around 4 minutes, but takes you trough hp from maya to baking and into substance painter as well as blending multiple alphas into one layered alpha
They used 2 UV sets and vertex blending on the smaller pieces. There was a polycount post explaining it on one of the environment art dumps http://polycount.com/discussion/comment/2533186/#Comment_2533186
If you have the exterior light brightening up the area where the organ is, then that's adding to your focal point. Your focal points need to blend into the scene naturally remember :)