1. The ticket was issued by BA as I stated in my original post (125xxx) -- 125 is the BA namespace for those of you who are unaware. That was a business class ticket (I) with the 1st S7 segment in Y. The checked bag was tagged through to BOS.
2. Russian passport holders do not need a transit visa for airside transit in Schengen when arriving from an international destination and connecting to an international destination. They do need a visa if there is a "domestic" Schengen segment included in the itinerary (e.g. SVO-AMS-CDG-JFK).
3. My colleagues holding Russian passports traveled on similar itineraries and they didn't have to cross the Schengen border in "normal" German airports (FRA, MUC).
4. I made connections in Stockholm on *separate* tickets (SU-BA in both directions) without luggage and I was offered an option to do airside transit without crossing the border between terminals which I didn't use because I wanted to go to the lounge. The answer to my question about airside transit in TXL was "Englisch nicht" or whatever the correct spelling is.