Notes, Notings, and Common Refrains

The "lustre of midday" bit seems to be striving to prove the plausibility of the story: yes, it was night-time, but I could still see what was going on, and here's why. Then there's the reporting of the sounds of the reindeer landing on the roof, which is also plausible.

I'm going to argue it both ways, and say that he could have heard St Nick's boots crossing the roof to the sleigh, and then heard the whistle to the team, and maybe some hoof/runner scraping sounds which told him they were taking off. And depending on the planned flight path, the reindeer might not be trying for altitude between stops, and the narrator could have seen them flying low almost as soon as they'd cleared the eaves of the house.

On the other hand, all the most plausible parts of the story come early on, when the dream might still be working hard to convince the dreamer... and then once the dreamer's brain is on board with the concept, details start to slide. And the fuzziness on what room we're in is the strongest hint in the direction of your theory.