2) A plea to God to save him and be his refuge.
3) He knows if there was injustice on his hands, if he had stolen or harmed an innocent, then he would deserve the treatment. He admits no guilt though, since he had done no wrong.
4) No; he was just asking God to act.
5) His righteousness and justice.
6) David is willing to be judged for his own righteousness. He trusts in God for his defense.
7) I think it's humans, and they will be subject to God's justice? (I'm uncertain I understand the question and quite certain the Olde Enlilsh has meh stumped.)

9) One pregnant with evil, and one who digs a hole.
10) It doesn't fight fair, and even when you haven't done anything wrong, it'll still come after you. And, in the end, it always fails.
We can lean on God and trust in Him when we're getting castigated without cause, and trust Him to work our justice.