I know that we can’t help the situation by simply making up some evidential categories, language isn’t that flexible, but we can at least discuss the options and reveal specific obstacles. Full-blown attempt at directing linguistic evolution isn’t feasible, but as far are long inferential chains are being built and learned and used and relied upon, why not try and make use of it?
I suspect that it might be possible to steer the discussion to creation of certain keywords dangling on the end of chains of inferential reasoning, that would later serve as evidential qualifiers. Some of the top-rated comments come from the irrationality game thread, and they’ve been edited to reference “irrationality game”, which serves as such a qualifier. “Counterfactual”, as in “counterfactual muggling” does not only derive its evidential meaning from general English usage, but also from it being heavily used in arguments of certain king here on LW.
I want to propose two more possible solutions.
First, as I initially assumed, wards that Quirrel cast at Mary’s are rendering them undetectable to patronus communication. That way, if there were no second Harry in Azkaban and third Harry on the way back, McGonagall’s patronus wouldn’t be able to contact him at all.
Second, for all we know about patronus’ methods of travel, it might get autonomously dispatched directly to target’s location, which is normally unique. There are no conservation laws that prohibit patronus splitting into two independent messengers, as there are no conservation laws preventing you from having two mirror reflections, or two acoustic echoes at the same time; and reflections and echoes can be interacted with in magical ways in Potterverse. That means that all two or three copies of Harry can get the same message, give non-conflicting answers, and McGonagall won’t suspect anything.