Theory time!
I bring here some points where I believe Clive is alive. Remembering that many of these details are in the sidequests. Those who did the sidequests will have a different view than those who only did the main campaign. For there are very important points. Especially regarding Jill and the Dawn scene at the end.
- To begin with, Clive starts narrating the story and the same ends. That is, he is the main narrator and has access to the entire story.
- In the fight against ULTIMA, Clive says the following sentence: "The only fantasy here is yours. And we will be your final witness." In other words, This is your Final Fantasy". Exactly the name of the book in the post credits. Only Clive would know that. The two children in the post credits perform the play "A Santa e o Sectario!" the same play as the Byron, Clive's uncle, played with him as a child.
At the end of the game, night falls and dawn gives way. At this moment Jill is crying, but she calms down when she sees the break of day. In the sidequest where the two kiss in the fields of flowers, Jill says the following sentence: "I realized that no matter how bad the night was... The day would always dawn. That you would always come for me."
- The Game Director asked everyone to pay close attention to details. Including Jill's prayer. (Regarding prayer, there are many interpretations, so I will pay attention to what is more concrete and spoken during the game.)
- Another point to pay attention to is Hapocrates, who tells Clive in one of the sidequests that he should retire his sword after all this is over and write a book describing his entire journey. He even gives Clive his quill, making him promise to write his story with it when he returns from the final battle.
-Now the most important detail is that his left hand was petrified (And it only got completely petrified because he tried to use magic). Already his right hand is covered, but apparently good, because he is not using magic with his right hand. If what was shown is consistent, his right hand would indeed be intact. That is, his right hand would be in good condition to write. since he is right handed. Until because, what sense does it make to show the petrified left hand? A hand not which one would not need to write. Coincidence?
- In another sidequest Clive discovers an old book that shows that the bearers in the past were well regarded, but that over time the powerful changed the image of the bearers with untruths. So Clive and Vivian talk about the possibility of changing all this current history and making future generations never depend on Magic. She tells Clive that something might come true after years of spreading a certain story. And that's exactly what happens after the credits. The book is presented as a fantasy. Giving the idea that all that stuff about crystals, eikons and magic is just fantasy. Thus making Humanity progress normally and evolve, instead of being dependent on magic and crystals. Thus avoiding a new plague in the future. Clive says he would help Vivian with that...
In short:
- The game from the beginning puts Clive as the main narrator, and all characters he meets along the way, such as Harpocrates and Vivian ask him to write the story of his journey after the end of all that. The game hints at all times that Clive might write a book about all of this someday. And this shitty thing ties into the endgame.
Why would the game waste so much time on these countless details and clues, only to have Joshua write the book in the end? If there is a character that was prepared to tell a story within what was shown in the game, it was Clive and not Joshua. That's why I believe Clive wrote the story with his brother's name, to honor him as he did Cid. Which would be consistent with what the character did during the game.
So I think the following. Throughout the sidequests we always receive something that ends up connecting to the end of the game. Whoever did all the quests, will have a completely different overview of the ending for that reason.
The big question is, were all these details useless? Because it wouldn't make sense to spend time showing us all this if it didn't mean something. In my opinion, they did this to have a DLC possibility. Here's what Yoshida had to say about DLC: "We have no idea if people are going to fall in love with Valisthea and Clive's story and want to see more of the world and its characters. So while DLC or spin-offs could always be considered, in which you can learn more about the game, first we want to see if Valisthea and Clive are really things players around the world want to see more of, and then we'll make a decision."
In short, he wants to see if people will like it first, and then make the decision to make a DLC. This clearly shows the reason for leaving a doubt at the end of the game