[chronotrig]

Spoiler for more reasoning:

All in all, a really nice and massive summary, pero. You spotted quite a few things I missed. Just a few things I found interesting...

Chapter 2

There was never any proof that Ronove exists or that anyone was impersonating him. After he first arrives in chapter 2, Maria doesn't seem to be there anymore, so no 'real' person ever saw him.
Actually, this scene brings up an interesting point. From our point of view, Beatrice give Maria the letter, and Maria leaves before Ronove and the ring show up.
This proves that the seal on that letter is a fake!
Either it was obviously unnecessary magic(this is the third game, we know she has memories from the previous 2 games because she talks to Battler in the same scene, and she knows that Kinzo always gives her the ring to start the 'game'), or the entire situation is nothing like what it seems.
This doesn't actually prove that Beatrice isn't who she says she is, but it gets pretty close.

Chapter 5

Don't give up on that theory about Rosa telling a fake story yet. It all depends on how literally you interpret the red text.

That line Beatrice uses to claim that her old self is dead...
Doesn't mean anything but 'Certainly dead', and in the previous sentence, she mentions 'doesn't that look dead to you?' (emphasis added).
If, for example, a really stupid crab was lying under or alongside Beatrice, this sentence might be referring to that!
And just because she can talk about this part of the story in red, it doesn't mean that the whole story was true, or even that the scene we and Battler see is exactly how Rosa remembers it.
It could all be an illusion created by Beatrice to fit with Rosa's story, whether that story itself was true or not.

But, if we assume that the red text was referring to Beatrice's, and moreover, was referring to the fact that she was dead at that time, then that could open up the biggest weakness of the red text we've seen...

These people in this 'other world' seem to be able to stop time for nice, long conversations...and in this scene, they seem to be able to shift at least their perspective back into the past. If Beatrice was saying she was definitely dead at that time, then the red text follows the current time frame.

What if they didn't shift back to the present when she said that there were no more than 18 people? In other words, that red text could have meant 'There were no more than 18 people on the island when I died'

In which case, who cares?