A president qualifies by winning a majority of the votes of the Electors.
If over eight years challengers to a candidate’s qualifications are unable to convince the courts, congress or 50 states’ Chief Elections Officials of their allegations, then there is no other recourse that I can think of.
American voters had the opportunity to rule on Obama’s qualifications, twice.
This tosses the "Natural Citizen" requirement out on it's head. Need I remind you that the Supreme Court opinion to interpreting the US Constitution can be summed up as "The Constitution may not be interpreted so as to render any of it's clauses as having no effect."
Ergo, it is not just about the electors. The other constitutional requirements must also be met. If the man has not attained the age of 35, he's ineligible, and it doesn't matter what the electors or the public has to say on the issue.
American voters had the opportunity to rule on Obama’s qualifications, twice.
Well this is not true at all. Firstly they were very badly misled by the media and various lawyers on the correct understanding of the issue, so they didn't really get to weigh the question with the appropriate due diligence.
Realistically, the voters are mostly followers, and if enough of the right sort of people tell them something, they simply buy it, and will act on their misinformation.