he could conceivably be a nazi collaborator
Bab el Ehr is in league with Rastapopoulos who supported him through the Arabair flight company and supplied weapons through Dawson. The idea that either of them would be in league with the Soviets is very unlikely.
According to Wikipedia, the Estonians had a very mixed history during WW2. The Soviets took over in 1940, only for the Germans to "liberate" the country in 1941. Many Estonians joined the German armed forces in response to Soviet atrocities committed during the occupation (Hitler or Stalin, what's the difference?).
However, many Estonians also served in the Finnish armed forces against the Soviets and it seems that many also got as far as Britain and served in the Royal Navy and RAF against the Germans. It follows that Skut may have been one of them and if he had flown a Mosquito during the war then flying them as a mercenary some ten years later makes sense.
I doubt if Herge would have given a sympathetic character like Skut a dubious background such as being a Nazi collaborator. Other such characters are shown to be on the side of the villains: the submarine commander in "Red Sea Sharks", or Hans Boehm, the sunglasses-wearing pilot in "Flight 714" or Doctor Krollspell from the same story.