In your case, you cannot pre-select seat at booking because your flight is tomorrow, so is now under the control of the departure station (JFK) and so, seat allocations/changes is now only possible at check-in (OLCI or at airport).
For the CDG-NCE leg, in any case, seat allocation is only possible at check-in.

As your flight is tomorrow, you'll be able to OLCI very soon today.

For the type of seat, you are correct : no equivalent of EC seat on AF. You can choose a seat plus at OLCI.

Just a question : why didn't you fly the non-stop DL JFK-NCE ?

Even if you change your flight, do not expect miracles in term of seating whatever the airline and flight you will choose. 24 hrs before departure, all these flights are usually very very full.