Does that tweeter come with a crossover (cap) or does the factory wiring harness already have a crossover built in?
There is a cap built into the factory tweeter. You can probably remove it and re use it.
Like @Chef_jpd said, the factory tweeter uses a a cap, so you would either have to reuse that or be running an active crossover to use the SEAs tweeter.
This actually helps a ton! Would you still happen to have that stock tweeter and bracket laying around to take a measurement of the width of the bracket vs the width of the tweeter? I definitely don't mind the idea of taking the bracket out as long as the OEM grille doesn't require it to mount to the pillar. It's still weird to me that even the tweeter you are running shows a 46mm width at the base, and 53mm width at the face, while the Focal 1.5" tweeter shows 42mm at the base and 47mm at the face when used in the same mount method. It could be slightly possible that this ends up being a direct drop-in when used without any of the mounting methods included.
The stock grill just pops in with some tabs and has no contact with the tweeter bracket. I wouldn't get too hung up on the dimensions because once you remove the bracket, you don't reuse it unless you want to go back to stock. In any case, I took the measurements below;