What happened to SB (story)?

[Jandor]

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I would guess the game is more canon than the lore book, especially when the point of contention that started this conversation is them mentioning your adventurer friends a lot in Stormblood, which came out after the lore book. It just doesn't make sense for the extra out of game source to take priority over the latest update to the game itself.

As for never seeing other adventurers in cutscenes, there are two parts to that. The first is that you do see them, I think in basically every multi player event you participate in the victory scene at the end shows your adventurer friends celebrate with you. The second is that they are not shown in story cutscenes for gameplay reasons, your adventurer friends are obviously actual random people put together from whatever the duty finder can assemble, the cutscenes before and after the duty proper have no idea who will be with you or who you were just with. In the lore whoever you're running it with is a close friend, in reality they're some random people you'll probably never see again. Gameplay and story segregation at work.

Your adventurer friends are basically like your chocobo companion, you're meant to just assume that they're always close enough that circumstances will allow you to go get them.

For Sohr Khai, the journal states: "Hraesvelgr commends you and your companions for your unwavering determination, and pledges to aid you in the coming battle with Nidhogg. Inquire of Alphinaud how he fared in his own trial."
For whatever reason Hraesvelgr was pretty chill with the idea of you bringing help to your trial.

Ifrit story mode is, as I already acknowledged, by far the most likely to have been solo'd. "Having failed to make you his minion, Ifrit attempts to exterminate you, but against all odds you succeed in overcoming the god-like being."
It never mentions any help and while it doesn't explicitly state solo in the same way as it repeatedly states you have help, that would be the default reading of it.

After Bismarck, your journal explains that you're still exhausted from fighting when Thordan and the Ascian take the key from you. "still weak from your struggles against Bismarck and Igeyhorm, can only watch, powerless to stop them."
You're adventurer friends are good, but your character is still the strongest one, they're your team, you lead them. If you're too exhausted to fight they must be on the verge of collapsing.

Essentially as far as I can tell in game, it either doesn't explicitly say either way, or it says you have help. They rarely make a point of shoving it in your face that you had help, but some of the dialogue and a lot of journal entries show that you have it. I personally think the point of not shoving it in your face is to allow players the freedom to for the most part explain it how they like.

If that's not the end of it then lets agree to disagree, not the end of the world. How you choose to interpret your characters story has no bearing on me and how I look at mine none on you.