The log of TW909 was the solo flight of Bomber Commands C-in-C, Sir Norman Bottomley. My impression from that log is that on their return to the UK, they did not actually land at St Mawgan but were diverted to Prestwick - not exactly nearby. Why isn't explained.
35 Squadron A Flight departed Lagens on the 25th August, but I think they left late at night so arriving St Mawgan early the next morning, the 26th. Likewise, B Flight probably arrived in the early hours of the 27th. The whole 15 aircraft then departed St Mawgan on the 29th.
I have some times from my father's Flying Log:
July 9th, 1252 arr St Mawgan
July 10th 1021 dep St Mawgan
August 27th 0700 arr St Mawgan
August 29th 1000 dep St Mawgan
He was in B Flight.
The paint scheme, white and black, was original meant for Tiger Force, the proposed RAF aerial attack on Japan at the end of WW2. 7 Squadron, like 35 Squadron, was earmarked for that operation and so, presumably, was also painted those colours. I would assume that the 16 Lancasters of 35 Squadron that went on Goodwill were the Squadron's own aircraft. There's no mention of anything else in the sources I have anyway.
You could find what 7 Squadron were doing at St Mawgan at that time by looking at their Operations Record Books at The National Archives, Kew (if they exist).