He also was a Canadian citizen and could have been a Cuban citizen if he wanted.
If one can be anything other than a US citizen at birth, one cannot be a natural born citizen
Absolutely not so. You need to study up. So-called dual citizenship does not and cannot have anything to do with natural born citizenship since it is out of the control of the united States who other countries consider their citizens.
No court anywhere in the United States would countenance such a thing for even a moment.