As always your encyclopedic answers floor me! Thank you for all the details--is this from Hoepfner? Foucart has a map (small one) of Biezun as you probably know--but being only the French side of the story it's hard to tell what really transpired.
Yes, Foucart has Bessières' dispatch from 23 December that says:
"The Prince of Ponte-Corvo had announced to me that he would arrive the same evening at Biezun and that he sent two companies of infantry to it.1
1.4 companies of the 6th Light, 2 voltigeurs and 2 carabiniers, under the command of Captain Roussel of the carabiniers (letter of the Ney marshal of December 30, 1806). The 6th Light belonged to the 6th Corps."
Your explanation that Bernadotte was in overall command explains, all the dispatches referring to 6th Corps as belonging to him. Though as you say there seems to be a disconnect between two companies that Bessières says were coming and the footnote which says four companies.
He also notes that Tilly arrived in the evening having ridden to the sound of the guns with the 4th Hussars and the 5th Chasseurs, leaving the 2nd Hussars at Racionz to maintain communications with the 7th Corps.
Again, my humble thanks.