*stops squealing and jumping up and down at that username* ...though I have to say I really prefer Nemuro to Mikage. Even if he dresses like a dork. Mmm, creepy levels of intelligence mixed with extreme repression.
*cough*
Beyond Pink (a Mikage & Nemuro shrine) has an analysis of the similarity between the painting and that scene - I think I've seen it on another site too, but I'm not sure.
Anyhow, that one's
http://www.bunnybeth.net/beyondpink/ - and the analysis itself is
http://www.bunnybeth.net/beyondpink/celeste.html (The section you're looking for in this case would be <i>Olympia</i>.)
I remember one key factor was that - if you notice - the cat at the foot of the bed is missing. As seen earlier in those eps, a black cat was a symbol for Tokiko and the family unit of her, her brother, and possibly Nemuro (at least he wishes ;P). One of their theories pertains to that.
And yes, the fake Mamiya is basically Mikage's sidekick. Though I haven't watched that scene in a couple of months, my guess is that the absence of the cat plus Mamiya acting as Nemuro's sidekick would mark the beginning of Mikage's building his little dreamworld - Tokiko is nowhere to be found (she was of crucial importance in his real past, but in his "frozen" world she doesn't even exist), and real Mamiya didn't want "eternity" while this one is asking for it. What's more, a) real Mamiya was deathly ill by this point and b) he never acted like Nemuro's sidekick/slave.
So. *cough*