"Gvardeyskiy Preobrazhenets" and "Preobrazhenskiy Gvardeyets" (Гвардейский преображенец; Преображенский гвардеец) are functionally synonymous. One is a member of the Preobrazhenskiy regiment which happens to belong to the Guard, and the other is a Guardsman who happens to belong to the Preobrazhenskiy regiment. We chose the one we did so that the "guard" prefix would be identical to the ones used for all the other guard units and would be more intuitive for players. Also keeping their elite status on the left-hand side lets you quickly parse your troop list and see all the Russian units under "Russkiy" and then all the veteran and guard units under "veteranskiy" and "gvardeyskiy" respectively. I agree "Leib-Gvardii" looks better though, but it seemed at the time better to keep all the prefixes adjectival and avoid the genitive case. For some reason. Another consideration was to keep the names as short as possible, so using the noun derived from the Regiment's name was more efficient. At the time the mod is set, the regiment consisted of 16 grenadier companies (4 battalions of 4 each) without any fusiliers.
As for the veteran units, they don't represent Guard regiments, so it wouldn't make sense to change their names as you suggested.