Bonjour Frimousse!
The second officer isn’t General ADC Dedulin (Palace Commandant 1906-1913) but General ADC Baron Vladimir Borisovich Fredericksz, Minister of the Imperial Court (1898-1917). He served in The Horse Guards (Garde à cheval) and is wearing their uniform on this photo, along with the order of St. Andrew (light blue sash), which very few general officers received.
A photo of General Dedulin can be found in General Spiridovich’s memoirs “Les dernières années de la Cour de Tzarskoié-Sélo” (part. 2).
More about your other interesting photos later.