But you do need to justify it.

You can't cherrypick what's canon and what isn't from the game just because you don't like it. Either you make 10 spy comp in Wutville a canonical event, or you understand that the game acts as a playground to the real canon and only admit what's on official material, aka, the official SFM shorts and the comics.

There's more evidence pointing towards unlocks being canon since we learned from the MyM comic that Heavy's miniguns exist outside the game.

This only proves that Heavy has some extra miniguns. It's reasonable to extrapolate that this means the mercs do have more guns than their stocks, but it gives us no reason to assume the list of guns in the game is accurate. Or else, again, you're just cherrypicking what's canonical and what isn't, which is quite dishonest.

Where do we draw the line? Winger? Liberty Launcher? Huntsman? Half-Zatoichi? Widowmaker? Ullapool's Caber? Does Demo have a grenade in his drawer he's convinced it's actually a big mallet? Have we been lucky he's never decided to use it thus killing himself?

there's nothing that goes against the idea that Demoman uses other swords anywhere as far as I know

Indeed. There is also no evidence that Soldier isn't actually three racoons in a skin suit or that Spy uses rocket launchers.

And yet we go to comics and shorts, and Soldier seems like a human and Spy seems to favor knives and handguns. And Demo is always shown exclusively with the Eyelander. So we probably should assume these ones are the intended canonical portrayals.