If incorporeal creature is surrounded by 9 corporeal creatures, can it move through them/their sqares? I only seems to find mention about moving through solid objects and moving through water. I mean human body is a solid object and 70% water, but is there some explicit rule about this?
Incorporeal creatures can fly most of the time too, meaning 8 creatures surrounding would not be enough.
Does that somehow interacts through overrun/acrobatics from mechanic points of view?
Overrun, no, unless the creature is intending to overrun its foe. (Which seems odd--a lot of combat maneuvers make little sense if one foe is incorporeal.)
Acrobatics, yes--the creature still provokes attacks of opportunity unless it uses an Acrobatics check to move through threatened and occupied squares. That check will likely be impossible with so many foes threatening it.
Sure, but it still provokes.