We seem to agree that no Orc would willingly bring the Ring to Sauron.
The question then is what has happened at Parth Galen from Mordor point of view?
Sauron sent 40 orcs (IIRC) under Grishnakh to divert twice as many Orcs from Isengard and Moria from carrying two captured hobbits to Isengard and to make them turn to Mordor instead.
The two hobbits, as far as Sauron could suppose
might have been carrying the Ring. Yet he sent only orcs. The available Nazgul, however, was not allowed to cross the River.
Why such strange orders? Could it be that Sauron himself was not informed of the happenings in Rohan?