Advantage does not stack.
The man on the wall would need line of sight and there is at least partial cover. Since his move ended with a dash, he couldn't fire this turn. He could guess but shooting through a tree is poorly advised. I'd have to look into that. I would give you full cover if you are on the other side of a large creature.
Flanking works for those who are in opposite positions, you must be roughly on the opposite side of at least one other ally. A third who is 90 degree off would not get flank but I am not too picky about that if you had unused movement because there's no penalty to move to that position as long as you don't disengage. If you say you flanked, I would take it to mean you moved to a flanking position and nove you.