In Japan a lot of older or tiny apartments don't have machines, so it's not uncommon for small places to be hidden around. They're also commonly used for washing bigger things a small home washer and drier couldn't, like futons or large duvets.
Why not? It seemed pretty clear from earlier chapters that as long as they were together, they were happy. Calling it exclusive now and maybe something a little more doesn't seem like that far of a stretch from what they had.
23 chapters certainly isn't a bad pace but it sure felt like a long time.
What I can't exactly figure out is it that she is trending more towards asexual or has she always just ignored that she liked her. Probably will see more in the future chapters but I'm interested.