The assertion that a PIC, whether in general terms or in this specific real incident and situation, has such obligation as to attempt to wrest custody and control of a deplaned passenger from the authorities of the jurisdiction to which the aircraft was diverted -- this assertion seems quite novel.
One point that, imho, is important to note here is that the interests of PICs in this general subject area typically flow in the exact opposite direction. In other words, how to get local authorities in the jurisdiction where the aircraft has landed - whether by diversion (by the PIC's discretion) or as scheduled - to exercise legal authority over an unruly passenger who the PIC either has caused to be deplaned or requests the local law enforcement to remove from the aircraft. The Tokyo Convention of 1963 (much longer formal title), and Montreal Protocol of 2014, address the problems, or some of the problems, of jurisdiction with regard to unruly passengers - but in the context of getting them off the aircraft and/or prosecuted, not retrieving them for continued transportation after being wrongfully and unlawfully removed by the state where the aircraft has landed. Unless I've gotten really rusty on some main provisions of Air law, that is......
As to diplomatic incident causation.... somehow I think, stretching my imagination to the breaking point as if this SLF/attorney were a PIC and in this fact situation - I think getting the heck out of Dodge would be what the estimable Mr. O'Leary would want, demand, and expect.