To answers the questions asked:
-Link can weild the sword of the creator, but cannot use it's full power, and would eventually get turned into a mindless beast set on destroying stuff.
-Shulk could depending on when in Xenoblade it happens, though to be specific, probably not until Zanza left his body. Or maybe Zanza being in Shulk would allow him to do so, since he's a god.
-Byleth can weild the Monado like Dunban does before the story starts, aka it would hurt them to use it and they wouldn't get visions. And if we're talking about the Monado 2 or 3, then I'm pretty sure it's just a no.
Yeah, in most cases, it would probably be a "no", but even if they reach that conclusion pretty quickly, there could be some interesting arguments made. Since the reason why the Monado works for Shulk is a huge spoiler, I don't think the characters should really know much about it, with the most they can come up with being that it manipulates Ether (which they could only tell through Pyra/Mythra since they could detect things like that). They could also theorize that Palutena could wield the Master Sword due to being a goddess of light just like Hylia, and
maybe Pit as well. This could prompt Pit to ask Palutena, which would likely end in Palutena offering to make him a Master Sword; All she needs is his hair or something else that would make him immediately reject the idea.
There could also be a one off gag where they're like "I think I've seen Kirby wield [X Weapon]." "Yeah, but I think Kirby can just wield anything."
The whole interaction gets bonus points from me if a bunch of characters join in and it devolves into a game shop argument where everyone has an opinion and nobody can agree who's correct, then one of the characters that they've been talking about arrives, and everyone demands that they settle the argument.
EDIT: It would also be funny if someone really random like Duck Hunt could use it for no explained reason.
