1) Northwest opens their seat maps 90 days prior to the flight date. People booking within the 90-days usually will get their seats at the time the reservation is made. If you booked earlier than 90-days, you have to go back to your reservation if you booked on line or call NWA if you booked over the phone and select the seats you want. If you do this early (like on day 90) then you should have no problem at all getting 3 across plus the aisle across. Keep in mind that if you do call on the 90th advance date that you can't book the return seats until 90-days befor the return date.
2) Each passenger can bring on a carry-on PLUS a personal item such as a purse or laptop computer. If there are 4 passengers in your group, then you can have 4 carry-ons.
3) Yes, Northwest does change flight times and can cancel flights just like every airline. The reality though is that usually flight times are changed a few minutes here or there, not significantly. But there can be significant changes too. If you're booking 6 months before departure there is more of a chance of changes than if you book 30-days before departure. I recommend you check your reservation a month before you leave and then again a few days before you leave, just to see if anything changed. Sometimes only the flight number changed, sometimes the departure time changed, sometimes the arrival time changed, sometimes all of them changed!
4) Not a whole lot more to know. Do advance check-in for your flight the day prior to departure and print your boarding passes; that saves time at the airport.
BobK/Orlando