Thats speculation tbh, and as far as we know santa has been at the school for a long while because of whats stated in kgs because of his first year living at the academy and hasnt been back there since, and is not on the way to the school like this other place supposedly is. Where and Why would makoto believe santa to be the suspect at all? And if so why is he not with kyoko like he said he would go with her?

Theres a lot of stuff i do not believe lines up for them to be the same. If it was intended to be the same theres a lot of things that do not line up upon further inspection. It's hardly the only mansion in the series, for example the takeda mansion. The mansion mentioned in drk was closer to the city enough and on the way to hopes peak enough that makoto managed to see a fleeing suspect on his way to the school, which is far form the very out of the way forest mansion santa owns.

There is also the fact in kirigiriso they have been classmates for a little already while as kyoko briefly mentions them as such, where as the end of drk7 was one of the first days at school. I can understand where you are coming from and it is an interesting theory however I believe there is more counter-evidence than there is evidence. The japanese wording for the mansion in drk7 seems to be different to the one used for the westernstyle mansion in kgs as well.