“If John Quincy Adams would have been born en route to or from the United States or even if Abigail had delivered him whilst in Great Britain or France, would he have been eligible to later serve as POTUS? “
If he was not born on US soil, he would not have been a US citizen and he would not have qualified to be POTUS.
However, he was born in Quincy MA on 07/11/1767, before the adoption of the constitution and is covered under the clause, “or a Citizen of the United States, at the time of the Adoption of this Constitution”
So, why the hypothetical question?
Oh, just to see how many people would reply with something entirely false and inaccurrate, such as this:
If he was not born on US soil, he would not have been a US citizen and he would not have qualified to be POTUS.