Just as I'd hoped, Morten Lindberg of 2L Recordings read my review below of Souvenir Part II and clarified the orchestration. It is NOT a mixed voice orchestration. Mr. Lindberg replied:
"Thank you, Marc! I can confirm that for the recording of this Part II the orchestra was not in mixed voices. Here's the drawings of the stage layout and session photos:"
The notes accompanying this diagram say the following:
"Preparing for recording Tchaikovsky’s “Souvenir de Florence” with TrondheimSolistene next week. Originally composed for a string sextet, Øyvind Gimse has expanded the score to chamber orchestra. Our planned stage layout will hopefully capture the dynamic between the sextet in the inner circle and the full orchestra surrounding them in an outer raised circle."
— with Beatrice Johannessen, Rolf Hoff Baltzersen, Dpa Denmark, Ole Wuttudal, Øyvind Gimse, TrondheimSolistene and TrondheimSolistene at Lindberg Lyd A/S.
For comparison, here is the layout for Souvenir Part I, which is indeed a mixed voice orchestration:
Thanks for the clarification, and congratulations on such an amazing sonic achievement!