A couple more articles that may provide a few clues as to who flew these missions.
In the 4th paragraph of this link is a description of a similar flight (or the same?) done in late June 41 with Harry Hopkins (Roosevelt's personal representative).
https://www.historynet.com/book-revi...-wills-ww2.htm
The paragraph gives the name of the pilot, Flight Lieutenant David McKinley.
Here is McKinley's obit, which mentions that after a short spell in 210 squadron:
http://www.rafcommands.com/archive/01245.php
" he joined the North Atlantic Ferry organization ferrying Catalinas from the USA to the UK. In 1941 he flew President Roosevelts personal envoy Harry Hopkins to Archangel for an assessment of Stalin. Flying a Catalina from Invergorden, Mckinley delivered his charge after a flight of 20 hours and was fortunate not to be intercepted by the Luftwaffe off Norway."
Note: Invergorden is on Cromarty Firth.
Here is another link:
https://s3-eu-west-1.amazonaws.com/s....%20hendry.pdf
(See the middle column at the bottom)
Which mentions that in July? 1941, a Catalina from 209 squadron and another for 240 squadron flew Sir Stafford Cripps and other government officials to Russia and returned with a Soviet Military Mission.
In the 209 squadron reports for July 1941 it is noted the F/Lt L. Fox returned to the squadron on 8 July after a trip to Russia.