Most reviews I’ve seen of Adaptation sum the final act up thusly:
They don’t mean this kindly, and I think that’s due in part to taking the film at face value. Adaptation is a self-aware film that doesn’t make a big show of how self-aware it is. I only realized this more than halfway through, when McKee doles out the advice:
God help you if you use voice-over in your work, my friends. God help you. That’s flaccid, sloppy writing. Any idiot can write a voice-over narration to explain the thoughts of a character.
And all the narration STOPS. No record scratch, no indication of a stopping point, it just ceases. At this point in the film AND at this point in the screenplay Charlie is trying to squeeze out of his brain, he’s had the creative crutch of voice-over knocked out from under his arm. If you see the film’s script as evolving and shifting and changing with Charlie’s character, the film (particularly the climax) becomes a very different best.
By the film’s final third, Charlie is already writing the script as the tale of him writing the script. When he asks his prostrate brother, “How would the great Donald end this?” the events that follow are exactly that. It’s the same overwrought, bombastic nonsense that sold Donald’s cheesy (but entertaining) first screenplay. All the way up to the return of a lazy, amateurish voice-over at the end.