@WhimsiCat
Exactly. Nanami mom just connected dots. Kaoru voice, smell, build not to mention all other hints like them going to change room together, but him later not showing up in hot springs. Author hinted it quite often anyway. She is just happy as parent that her daughter found first true friend and probably first love. Also she knows that they both yet don't understand (she know how Nanami is social awkward) that they are becoming something more and just haves fun teasing them while playing along they little secret- and she checked if Kaoru is good person quite few times too.
Same goes with tease about Kaoru remembering his mother- hinting that he will have more romantic feelings now. I mean it is normal in family with good relationship that son will subconsciously look for partner similar to his mother and daughter will look subconsciously for someone similar to her father.
Aww. Our social recluse is doing her best.
It's not really a good sign when the cook goes "I've seen the trope where the mistakes were made, so I probably won't mess up" though.
The crossdressing is a fun bit for me, so I'm fine if it sticks around.
I thought the change was because she had initially thought that Kaori-chan was a nickname, but upon learning that it was “her” actual name she changed to the more formal one as a sign of respect.