It is difficult to conceive of a "guise" which could be devised by the "current administration" to "compel" a former U.S. ambassador to travel to a foreign nation for "questioning", due to the Privileges and Immunities granted to high-level diplomats; and the limited power of the Executive Branch under the Constitution or laws of the United States to force such a compulsion upon a U.S. citizen.
We would first need to determine the authority any Branch of the United States would have to make such an order, or "compel" a former diplomat and U.S. citizen to do anything.
The single organic Law of the United States that can immediately cite relevant to ambassadors would be the Constitution of the United States, Article III, Section 2 (in pertinent part)
The judicial Power shall extend to all Cases, in Law and Equity,
arising under this Constitution, the Laws of the United States, and
Treaties made, or which shall be made, under their Authority;—to all
Cases affecting Ambassadors, other public Ministers and Consuls;—to
all Cases of admiralty and maritime Jurisdiction;—to Controversies to
which the United States shall be a Party
where a party could file an action directly in the U.S. Supreme Court expressly seeking a specific remedy as to that diplomat, or as would be the case, the United States - as the questioning would presumably be related to the duties the ambassador performed while acting in their official capacity as a representative of the United States, not as a private U.S. citizen - though am not certain of the purported nature of the inquiry which spawned the proposal to "question" a former U.S. ambassador.
It should be possible to determine if such a filing has been made by contacting the Clerk of the Court and reviewing the Court Docket.