Looking over the two photos again I believe you are right about them being taken at the same time. I had always assumed #450 was late in MvR's career do to the huge rack of awards being displayed. However, his face looks much more like that of the confident young PLM winner than that of the wounded, war-weary commander.
Even if this isn't the case I'm sure you are right about his wearing the dress blues in both pictures because the tunic is a much darker material than the cap which is darker than the cape and the tunic botton that peeks out from underneight the right side of the cape is very shiny.
I suppose this means that the tunic is Prussian Blue with a Poppy Red collar and guilt buttons? Seems odd that he's wearing a feldgrau peak cap...
Another good link:
http://www.sanke-cards.com/index.htm