death to capslock

I think we can believe Dumbles that Tom got the idea to use 'Sirius' as bait from Kreacher (not directly, because as far as Tom knew Kreacher died in 1979 or so, but Narcissa passed the information on). Sirius commanded Kreacher to go 'out' the night Arthur was attacked (December 18th, according to Lexicon). Kreacher showed up again around the time Severus visited 12GP to tell Harry about the Occlumency lessons (January 11th, ibid). The first time Harry realized the location of the door from his dream was during the first Occlumency lesson. Hmm, didn't he actually see Arthur leading him to the court-room? And wasn't Lucius present there that day? I think the image Tom showed Harry earlier (starting from the summer) was from a memory of someone (either one of the DEs who works or has business at the Ministry or even a memory of Tom's in one of his incarnations) going to that door. Then in January he showed Harry Lucius' memory of Arthur bringing him to the court room. And at some point (could have started before the night in late February when Harry managed to get past the door if previous instances were also interrupted) Tom started showing the dream that was supposed to lead Harry to the prophecy shelf. So where did the imagery in the dream come from? It cold only have come from someone who had been past the door (because the images in the dream matched reality when Harry got to the DOM). So either from Rookwood, or Bode, or maybe Tom himself, if he actually got that far himself in some manner.

Did the image of Sirius come from a memory - whether from Severus or Peter (I find it hard to explain how it could come from Kreacher without Tom finding out he was alive)? Or did someone Polyjuice hirself into Sirius to act the part in order to create the desired scene?