How would one identify if one were operating from a location directly on the US/Canada border? Wxxxx/VEx or Wxxxx?
That feels like a very constructed question. In all likelihood, nobody would care and you could just pick.
What a judge would decide if you were fined by either spectrum authority and appealed that fine is only known to the hypothetical judge.
Whether or not you're in violation of code so that they'd fine you is in the end subject to interpretation by the spectrum authority, which might consider you on their "side" if parts of your activity do happen on their side. Which sounds likely, given that borders are infinity thin imaginary lines, and operators are not. Whether the responsible person in that authority seems that a relevant beach of code is, again, known to nobody but that hypothetical person.
Again, realistically nobody would know or care,
Also, reminder that sending your identification has a simple purpose: identifying you, and either choice will. It's thoroughly irrelevant whether you're 5m south or north of a border to radio propagation, so there is no practical advantage to being overly specific, either.
Well, if one follows https://docs.pota.app/docs/rules.html#park-boundaries-and-multi-park-activations -
Please be aware of the beauty in the simplicity of the rule “the activator and all equipment must be within the park’s boundary and on public property.” If an activator is straddling the park or state lines, they are, therefore, not fully within the boundary and are out of compliance with the rule.
So, you and your gear must be in one country or the other at a time, so you would identify as appropriate for the country/park you are in. This presumes the international border is the park border as well, so non-overlapping. Given the reference to "state lines" I will assume the US-Canada border is equivalent to a state line.