MichaelPIT, the 2 years guidelines are not the same between the CAA and the FAA. I posted the current guidance document from the FAA earlier in this thread.
If an infant becomes a child during the trip, the fare rules are calculated based on the age at origin of the trip. The seating rules for each leg follow the actual age at each flight.
None of that stuff is terribly relevant to this situation, where (as it has been pointed out multiple times) this comes down to a passenger name mismatch on the return flight that day.