I doubt it is ai. The same room with the fireplace can be seen in both the picture featuring it and the hallway shot where it is barely in view, but the same furniture, blinds, rug, wall molding, etc. is visible in both. The apparent discrepancy in dimensions is likely the focal length on the lenses used.

The last shot is from the sequel. Same house still. But I don't think that the after shot is necessarily the same room. The windows and fireplace placement doesnt line up.

House seems real. Here is a youtube short of a guy walking through it and an Architectural Digest article about the house, its history, and its transformation to the drab and now even more giant state it is in. I turned off javascript and was able to read the whole article. It also talks about how most of the filming was offsite, as you said.

Current owner says they wish to restore the interior.

Yeah, set dressers to a hell of a job. Not just a sound stage but the entire house was dressed to be Christmas colors. Everything is red, green, and gold. Because it's a Christmas movie. I doubt the house actually looked like this.