I dont understand why you would pay to pre book seats and then only be able to pick them a few weeks before you leave
Surely then everyone else that has prebooked also will get the option to do this and if it starts at midnight which normally happens then who is to say you will have a decent seat left when you wake up in the morning
In my opinion unless you are paying to book extra legroom/exit seats (which you can't if you have kids booked with you) then it is a waste of time
I have never pre booked normal seats as you turn up early at the airport and they will seat you together
As for your other question about the best place to sit on the plane, with the company you have chosen to fly with again in my own opinion, the best place to sit on that plane is in the toilet
I did this on a flight from Turkey to UK one time as they would not let me sit in the extra leg room seats that were empty even though we were over Izmir at the time unless I paid, I could not fit into the seat they had allocated me so I told the stewardesss that the toilet was now full for the rest of the trip
I sat in the toilet with my laptop for 4 hours and got through a movie and a few games and was very comfortable in my new wide extra legroom seat albeit without a window.
On another trip with on a plane that was switched to the same company from Vancouver I sat downstairs next to a toilet because I could not get comfortable in the seat.
I should have also known better but the price was unreal on one of their flights to Las Vegas from Manchester a few years ago and again could not fit in the seats comfortably so stood up at the back of the bus

with the stewards for 10 hours drinking jack and coke and having a good chat
Never again for me I would rather walk