Some ideas

You could try calling again, speaking with a supervisor if necessary.

Normally when there is a problem, DL first tries to reroute using DL operated flights, where it can easily open inventory if they are selling seats and sometimes even when then flight is already full and not even selling full fare revenue tickets in the cabin class. Next DL tries to use KLM or AF, due to the joint venture, before going to other SkyTeam. Usually they will not approve nonSkyTeam flights and will refund the ticket instead. IMO because of he tight relation NW had with KLM, DL might prefer to use KLM over AF in these situations. Keeping this in mind, find some routings using KLM and AF, with as much DL as possible, that you would want.

AF might release low award availability on the A380 closer to the flight date, so you might be able to switch to it later. Ask nicely for a fee waiver if that happens.

Do you and your mother want to be on the same TATL flight? That restricts the routings that are legal for both of you, such as nonstop CDG to West Coast for you.

Have you considered CDG or AMS non stops into DTW or MSP, followed by domestic connections for each of you? If you do this, avoid the 767-300s if possible. Alternatively, for nice planes and flat seats, have you explored LHR connections?

The A330s are nice new planes that look fresh and well maintained, especially in BE. The only disadvantage is the angled flat seats, which are still much better than the recliners or the AF angled flat seats in 2-3-2 configurations.

I would not worry about getting two seats together in BE. Usually someone is willing to switch, especially for an older mom and adult kid traveling together. Just get two good seats and offer like for like or a better seat to someone when they arrive. Make a reasonable polite request and do not do anything that could be interpreted as claiming the seat before asking.