I totally agree with what you wrote :

"The question of ranks within the Imperial Guard is not simple [...]Later the system became even more complicated.

I had had a look at the Quintin and the Six (as well as the Dictionnaire Napoléon) before writing the text above. I had seen that none of these sources mentions him as a general.

The only absolute source is his military record, but I can't have a look at that.

On the other hand, I don't think there is a single colonel on the Arc de Triomphe, even more, only about a quarter of all generals are mentioned ! (660 out of more than 2200)

There isn't a colonel at Versailles either. Even more, the Inscription above the plate where his name is mentioned, is " Liste des Maréchaux de Camp, chefs d'escadre, contre-amiraux et généraux de brigade"

Maybe there was some kind of posthumous "regularisation" ? This might explain why these later sources mention him as a general.

The Almanach impérial is not an argument, since it didn't include promotions during 1805.

I think the problem come from the fact that he had had his promotion inside the Guard.

The superior rank of the Guard was most clearly apparent when someone lost a rank entering the Guard, or got promoted when leaving the Guard for the Line.

Like you said : "The question of ranks within the Imperial Guard is not simple..."