There is no special security lane for US flights (unless you mean the secondary screening for US flights at the gate) It should be OK
/nothing to worry about.

Yes, to check-in at 10 PM for a 6AM flight is no issue. If it was me, I would spend the day and evening in the city and arrive IST around midnight or later. It takes at least 1 hour to get to the airport but the traffic in Istanbul can be bad in the early evening, so the later you you leave from Istanbul city the better.

No issue for lounge access.

Yes C/J class have access to priority boarding.