1. No, they are released at the airport. Row 7 is also blocked, I believe.
2. Yes, check-in as soon as possible, since if you are not elite you will have access to the "preferred" elite seats that you previously did not have access too. (It sounds like in this case they may be taken already.)
3. You can still change your seat after OLCI, go through Delta.com like you are checking-in again and you'll have the options to change seats. (**I am not sure though if you check-in and get a "SEAT REQUEST CARD" if you can come back and later get seats if they open up. Memory tells me that for some reason you can't. In that case you can call up and have them undo your check-in and then do it again.)
4. Yes. The bag still must meet Delta's carry-on size dimensions though to get through TSA.
5. You should be fine, my no status brother is on this run quite a bit and has never had trouble getting a seat even with it down to Y0 on day of departure.
Good luck!