1. Can I combine Grapple Dmg or Grapple-Tackle-Dmg with Adept Powers: Killings Hands, Critical Strike?
If you spend a Minor Action to activate Killing Hands then it will be applied when you deal damage to your grappled target. Previous editions made it clear that it is not just applied when you punch people with your hands.
Critical Strike is a passive power that will also be applied when you deal damage to your grappled target.
I also can't find anything prohibiting you from applying them both when you tackle your opponent.
2. I dont understand Break free action from grapple rules: Is it close combat+ Str. vs close combat Str. OR close combat +Str. vs previous attacker net hits?
As written it is actually a Simple Test, but I would borrow some intention from the Restrain attack and resolve it as an Opposed Close Combat + Strength vs. Close Combat + Strength Test.