Alamanach 1805 is an argument, because you will not only find there Morland as "major only", but also because you will find there another guard colonel: Couin, with the artillerie a cheval (he will not be general untill 9 mars 1806; Six does not give the date of his promotion to the rank of colonel mentionned by the Almanach however)
I agree with what you say except that we talk here about a general, and this high rank was not influenced by the organizational decrees of the guard who ordered specific relation between the guard soldiers and the line soldiers, including the officers.
The decree of 13 july 1804 says : "Les officiers et sous-officiers de la Garde impérial ont, a grade égal, le commandement sur les officiers et sous-officiers des corps de ligne, lorsqu'ils se trouvent réunis dans un poste pour le meme service."
It does not mean (yet) that the guard capitaine is chef de bataillon, he is commanded by line chef de bataillon, but he commands the line capitaines... and he is capitaine.
There is no change in this during the Empire as far as I know, untill the decree of 8th april 1815 :
"A compter du grade de major, les officiers, sous-officiers et soldats de la vieille garde auront le rang du grade immédiatement supérieur dans la ligne; les officiers en porteront les marques distinctives; ainsi, les majors auront le rang de colonel dans la ligne (ils pourront etre maréchaux-de-camp). Les chefs de bataille, rang de major, etc."
Nowhere is said that the rank of guard colonel corresponds a "line rank" of general; there were no line generals, no guard generals, there were only generals.
I really do not know why Morland figures among other generals at the Arc de Triomphe or in Versailles, and I can only guess it is because of some posthume honourable and most probably not official promotion...