Black shadow leaks, inconsistent shadows, jagged shadow lines


I read like 2 years of forum entries about lightmass problems and how to deal with several problems from Epic, from Archviz guys, Youtube and so on.
But I still have some questions.

What I get:

  1. Jagged shadow lines. I know I can get rid of these with higher lightmap resolution, but it results in much longer building times. Is there another way to get sharp shadow lines?
  2. Black edges. I know that I have to snap the UV tiles on the grid, and I did in Blender. Snapped to pixel on a 512x512 and separated UV islands with enough margins. But why are there still these black shadow leaks?
  3. Why is the shadow breaking at corners? And how do I get rid of these?

I managed to tame these light leaks with building a cage around the room. But these shadows give me a lot of headaches.
The cage is one extra mesh and the walls inside are just connected planes to increase the lightmap resolution for these walls.
Thanks for any kind of help or link to tutorials, tipps or tricks.