In the original 1956 novel the central character was Wing- Commander Roy Grenville. This was changed to Wing - Commander Roy Grant for the film. The script did make it clear that Grant was an ex - Eagle Squadron pilot. A number of Americans did fly for the RAF during WW2. After Pearl Harbour the Eagle Squadrons were eventually transferred to the USAAF but some Americans stayed in the RAF so it is not totally implausible to have an American leading an RAF squadron.
I agree that the Greek American actor George Chakiris does not look very Norwegian! However the casting of these two American actors (Robertson and Chakiris) was done to ensure the success of the film at the American box office. British war films with all- British casts tend to bomb at the American box office. The 1969 film "Battle of Britain" was the most successful film at the UK box office when it came out but it was a financial disaster in most other countries. That was why the American role in "Operation Market Garden" was prominently featured in the 1977 film "A Bridge Too Far".
I agree that the ending was ambiguous. In Frederick E Smith's 1976 sequel "Operation Rhine Maiden" it was made clear that Wing Cdr Grant had survived the crash and become a POW - in the film it is not clear whether he had died or just lost consciousness.
All the Mosquitoes used in the film were obtained from No 35 Civilian Anti Aircraft Cooperation Unit in Exeter which retired its last Mosquitoes only a few weeks before filming began. These civilian - piloted Mosquitoes were the last in service anywhere in the world.
A total off 11 Mosquitoes were used in the filming though only four were airworthy . Three Mosquitoes were destroyed during filming.
A few of the Mosquitoes used in the film still exist though none are currently airworthy. The B-25 Mitchell used as the camera plane still exists albeit in a derelict condition at North Weald Airfield in England.