On the 21st of June, Japanese defences on the southern fringe of Okinawa collapsed. The following day General Mitsuru Ushijima and chief of staff Isamu Chō committed suicide. Colonel Yahara was denied permission to commit suicide and presumably negotiated surrender terms with allied forces:
If you die there will be no one left who knows the truth about the battle of Okinawa. Bear the temporary shame but endure it. This is an order from your army Commander.
Where and when (latitude and longitude if possible) was the surrender signed?
So my assumptions about Yahara signing the surrender terms soon after the collapse were unsound. Reading the interrogation report included as an appendix of his book it is clear that instead Yahara managed to evade capture for several days before being picked up by local CIC agents. He remained a prisoner until January 1946 when he was repatriated to Japan and was not involved in surrender ceremonies on the 7th of September.