I mean Subaru was throwing up, which makes me think poison. If we assume the first reset in the mansion was caused by Subaru dying in his sleep by poison, or choking on his own vomit, we gotta figure it has to be Rem. I think Rem is actually the one who does everything, right? Including cooking.
Then there's the sounds of armour we heard clanking through the hallway. For that, I unno, a part of me thinks that's just like an enchanted piece of armour set to guard the house and kill intruders, which they saw Subaru as considering he was a new face. The maids might all have "forgotten" to make arrangements to prevent late night death. The fabulous guy, his name eludes me, is a real strong mage after all and has reason to zealously guard his property. Hell, he is constantly manipulating space just to protect his books, as well as having a guardian with them most of the time. He's also a bit of a creeper what with having his maids sit on his lap in the middle of the night and seeing his staff as his property.
For the change in routine, I think that was just Rem and Ram spiting him when he tried to push them along with that stuff rather than letting them get on with it.
It is interesting that Rem seems to have an inferiority complex. Maybe Ram is just the master's favourite and she gets shut out all the time.