荷解き is unpacking, so she's not "preparing for her move," but hasn't yet unpacked from moving. 結構前 is also a while ago, not "not that long ago."
片付ける can be cleaning up, but since we're talking about unpacking, it'd be more like putting stuff away/organizing it instead of leaving it sitting in boxes.
年俸 is an annual salary as in getting paid once a year and not monthly/biweekly. 年俸入った汚部屋 is probably a joke about how it's been more than a year since she moved and still hasn't unpacked. Literally something like a dirty room that's gotten its annual pay.
When she's talking about the maid costume, she says like "since it's cute, and it's okay even if it gets dirty, and it feels like I'm in cleaning mode."
For 素, I think maybe something like simple or uncomplicated might be better than bare bones. I'm not sure if it's supposed to really make sense. I think it's her making an excuse or explaining away their relationship as like just plain, simple friends and not something more complicated. (And he thinks back to cases where she's unloading on him or whatever.)
ぽいぽい is maybe like っぽい. So he says it bothers him when it's messy, she says kind of like "that does seem like you" and "I bet your place is really clean."