I think that Rose’s spirit is bound to the Mansion as a corporeal spirit; in a sense, she IS the mansion now.
As to Maren, I think that she might well be the key to the darkest secret of Legacy Manor: EvilTrace’s attempt to resurrect his dead wife Saria. I think he created a spirit of his dead wife through necromantic means and Maren was/is to be the host body.
The current year in the comic is 482. Rose has been dead for two years. And before you say , "but what if the date is just the time she was seneschal," Rose was nine years old when she met Saria and 480 minus 454 is 26 and her age as listed in her reference sheet is 28. So the date can't be just her term of service.
Considering how in the current timeline, she can instantly move anywhere and hear anyone in the Estate, I'd wager the year 480 isn't "technically" the date of her death, but the date where she stopped being Seneschal of the Estate and started being..."part" of the estate, through some possibly (definitely?) unethical magical shenanigans (considering, upon looking at the outside world through the magic portal earlier on, she muses she hasn't been outside for a while). But that's just my theory