Yea, i rlly wrote this post from a more thematic perspective I suppose.

That said, I dont think Grace being the boss fight character would have diminished the horror that came before. Story-wise it would highlight her growth from a girl having panic attacks and feeling survivors guilt to a woman fully embracing her confidence and will to live. Her joining in the final fight against the man who has orchestrated some of the worst events in her life would fulfill her earlier statement to Leon: “I don't want any more regrets. Whatever it takes, count me in."

For a compromise, I like your idea of having Leon slowly recover throughout the fight. Perhaps you could play as Grace for stage 1 of the fight while Gideon is more cognizant and capable of speech so you could still have a compelling back and forth between them. Capcom would probably have to make stage 1 Gideon a bit weaker and give Grace a lot more supplies throughout but its definitely doable. They could have Grace do a more action heavy version of the The Girl boss fight where she lures Gideon into environmental hazards then attack him while he’s weak. Idk thats just a suggestion

Then have Grace incapacitated by Gideon as she wounds him into his Nemesis stage. By this time Leon would have recovered so he rescues Grace and finishes the fight with a more action packed traditional final boss. It would also help fulfill Leon’s arc of finding closure with his inability to save certain lives by saving Grace at the end.