Some progress report:
Huge performance increase achieved by disabling SSAO.
Massive improvement in shadows quality! No flickering, no jagged edges.
What I did?
Spotlight angle: 90 ***
Clipping distance: 0.2
Light source position: pretty far in front of the lantern model
Disabled: Parallax, SSAO
Now, everything above sucks, but it gave, literally, massive improvement and I'm investigating how far back can I go with these settings before the shadows get screwed again. Stay tuned for some media.
I think the spotlight angle I used was 155. Some sshots may present one of the early WIP versions at much lower angle. Higher angle gave worse shadows, altough exact relation between the two is not clear to me.