On Titanic the switches controlling the lights would have been small boxes, not round like the heater switch you referenced. In fact you can see a light switch on the right side of the door in the Browne pic. Also if the space was as previously though the small corridor would have been considered a part of the public areas which would mean that the lights would have been controlled from a fuse box.
Regardless of what the object is, considering the given evidence, it can be said that the door did exist in my opinion. The most damning evidence seems to be the points i made about the panel and the dark corner. the white stripe that I identified as the top of the door in the original article seems to not be the actual top though. I think it might be a part of the double exposure of the picture or possible a scratch on the negative. Having a door there doesn't seems that outlandish either when you look at the history of the Olympic Class; the door was featured in Britannic's 1915 plan and was later added on Olympic. The modification Browne made to his deck plan just seems to top of a long list of evidence. Maybe even too long to be a coincidence? That's what I think.
Sorry for not answering sooner I was a bit busy with other stuff.