question

Your OEM B2B charger should certainly be disabled, especially if it's not working properly, but you'd want to get in touch with the dealer themselves to have that done or at least get a wiring diagram so you can figure out how to do it yourself, or perhaps poke around for some forums specific to your vehicle where others may have run into the same issues and might have furnished wiring diagrams or instructions.
Since the vehicle is a basically new model, I'd advise starting with the dealer first just to avoid any possibility of voiding your vehicle warranty through "unauthorized modification" or any other silliness.

Almost all B2B chargers should work fine if wired correctly. But if you want to replace it, that should be pretty comparable, just remove the old one and rewire.

You need to bypass the existing, you can't put 2 B2B's in series. Which B2B is installed? In Europe, it's mainly Schaudt, occasionally Redarc or Votronic which are installed.

But your dealer should be able to fix this, unless its a dealer which has no clue about what they are selling. (But in that case I would recommend going to a dealer which actually has knowledge of their products instead of just taking your money)