CONCENTRATE ON ONE PROJECTION: Give Dr. Strange a free action and choose an opposing character adjacent to a friendly Astral Dr. Strange. This turn, Dr. Strange can target that character regardless of range, squares, or line of fire when making a ranged combat attack or using Outwit or Perplex.

Since it says "regardless of range, squares, or line of fire", does that means he doesnt actually needs line of fire, so stealth or similar abilities that block LoF are useless against him?

Correct. No LOF, so no Stealth or even Hindering modifier. Also note he is not using the Astral to target through: once the Astral "tags" the opposing figure the Astral's work is done.
Effects that say "can't target more than X squares away" or similar effects will still effect him, though.

Yes, as long as you had a friendly Astral Dr. Strange adjacent to that specific opposing character when you gave Dr. Strange a free action.