I don't think Flying Blue is the right programme for the OP.

As mentioned above, As FB Gold, you would be treated only like Skyteam Elite Plus on other airlines. Your benefits do not extend to others on the same PNR.

The policy varies by airline, but Skyteam status does not guarantee free seat selection or access to preferred seats, and certainly will not extend to non status members on the same PNR. It sounds like DL Skymiles and one of their bank cards is a better option for you.