Excellent point! If 100% of Americans (and the EC as well of course) voted to make King Charles President, he still wouldn't be, since he's ineligible by law. That also would defy the will of the people but I think most of us are okay with that.
The will of the people is expressed in vote and then indirectly applied by the elected officials.
If 100% of Americans decided to make King Charles president, then they will amend the laws allowing him to do so.
If literally 100% of Americans are voting to make Charles III their President, that 100% must include Congress unanimously voting to approve the constitutional amendment to make him eligible to run for President, and all the state legislatures voting to ratify it. If any of them vote against that amendment, then your premise that 100% of Americans vote for him would be false.
He would most certainly be, since reality moves by itself and not by any rules written by men.