Is last year's Hajj valid despite the doubt that two or three pebbles did not land in the basin when stoning the Jamrat al-Aqaba on the Day of Sacrifice?

If the doubt about the stoning occurred after its completion, then the stoning is valid, and you owe nothing, because doubt after the completion of an act of worship has no effect. However, if the doubt occurred during the stoning regarding the pebbles reaching the basin, then you must complete the number until you are certain that seven pebbles have reached it, because a doubtful stoning is not considered valid in Sharia, and one's obligation is not discharged except with certainty.

If you left three pebbles or more, then you owe a dam (a sheep to be slaughtered in the Haram and distributed to the poor). As for leaving one or two pebbles, there are different opinions among scholars: some consider a mudd for one pebble and two mudds for two, some consider nothing, and some obligate a dam even for a single pebble.

If your companion is a just and trustworthy person, then relying on his word regarding the number of pebbles is sufficient for you.