I have a Conn 80A cornet and I know that Bix's cornets were longer than the traditional cornets from today. Besides that, I'm aware that a person who plays one of these instruments can easily switch between flugelhorn, cornet and trumpet. Nevertheless, I don't think that Bix played the trumpet because there is no account on that. Unlike Louis Armstrong who switched from cornet to trumpet sometime at the beginning of his Hot 5/Hot 7 recordings, Bix remained a cornet player. As far as I know, he even played the cornet when he played classical pieces such as Concerto in F or when he played for that trumpet master who told him that he shouldn't try to change because he had a God-given gift.
I don't want to exclude it entirely that the player on the Whiteman photo might be Bix. However, I don't think that this player holds the Bach Strad in his hands. Just have a look at the photos provided by Albert once again:
Considering the fact that the player on this photo doesn't hold his instrument in a diagonal position for the viewer, the front part of this instrument must be longer than the one of the Bach Strad on the 3rd photo.
I think as a drawer, you can see that better than I do. Just imagine a hand holding the Bach Strad. I'd be interested in your opinion.
In order to identify Bix on this photo, we should try to identify all the other players on it and look at photos of the trumpet/cornet players who were in the payroll of Whiteman when the photo was taken.