Gladly! 

I'm not sure how this would work in Arena, you can tell me that. In actuality though, in this hypothetical, none of these are possible for the following reason: footwork. This is the scenario, from the top, with your hypotheticals included. I just spent about two minutes demoing each with my brother -- it was very fun (though he died repeatedly):

Florian is in invite. Brennus throws himself with a gran passata, right foot forward. Florian intercepts and parries. He grabs his sword arm with his free hand. At this point, Brennus still has his left foot way back. It's discharged and so can't do anything. If he moves his left foot forward to recover balance, Florian stabs him. I determined that he could maybe punch him in the crotch, somewhat half-heartedly, but he'd receive a point to the face in response. He can't push himself back with his right leg because the grapple has connected. 

Maybe, maaaaybe, Brennus could push himself back with his right leg the instant before Florian's grapple connects, but his leg isn't fully charged because it's so extended, and at this point Florian could throw himself at Brennus in fleche or something and gore him. So Vordak's previous suggestion isn't instant death, but it's still not optimal. The bit where I said extremely skilled fencers could do such a thing turns out to be a different attack. 

Am I making sense? 

edit: Though there has been an edit and a post in the time I posted this, I think it still stands. The hypothetical is in the scenario that Brennus throws himself all out, right? If it's not, then the fight is not over at all.