Actually he’s not “being accused” of being a citizen of any other country, not directly. He’s being “accused” of not having two US citizen parents, which he and his staff readily admit.
Not hardly. There is a decision as recent as 1971 that says those born citizens, who are citizens because of 8 USC 1401, who were born outside the US are naturalized and not even covered by the 14th amendment, since they are not "naturalized in the United States". Basically the decision indicates that their "citizenship from birth" may be removed for failure to meet certain criteria. This is not true of person's naturalized in the US, unless it can be determined that they were naturalized based on fraudulently provided information.