What is canon and what is not (Spoilers)

[WhiteArchmage]

The general rule I think is that:

Anything in the MSQ is hard canon.
Sidequests are semi-canon until the MSQ brings them up and confirms them. Like Crystal Tower which has been deemed canon since 5.3 (technically speaking since 5.0, but with some Time Travel Tense Problem meaning that it's happened in the past but can still happen in your future, but 5.3 NEEDS the events of CT)
Omega, Alexander, and Coil are kind of canon in that the first 2 are not REQUIRED for 5.x, but greatly flesh it out.
Ivalice is generally canon as of 4.5, given that the Dalmascan Resistance shows up in an Easter Egg; and the Wandering Dramaturge from the Prima Vista is now in the Blades of Gunhildr storyline.
IIRC the only thing not SUPER canon are Hildibrand Quests... and even then I'm not that sure because the Scholasticate quests needed the ARR Hildibrand quests to unlock, and Ultros, Typhon, and Yojimbo have become part of the Gold Saucer...

However, technically speaking, the PC is not the only one of Hydaelin's Chosen (there are other people in the opening cutscene), but IS the only "Warrior of Light". The other PCs you run into are random adventurers who are probably Echo-blessed so as to fight primals but aren't THE WoL/D. The game, at least, doesn't treat their race/gender combinations as anything special (like Moonkeeper Miqo'te Males being rare, or Au Ra not being shot on sight in Coerthas pre-3.0); any Lore about it would be the RPers personal backstory, I'm guessing.

And traveling to/from the First... afaik only The WoL/D should be able to do that in both body and soul... any Loremongers are free to correct me on this one.