He's not intentionally being creepy by forcing his way in through the door.
In his stunted, 12yo mind, he's just giving the key back.
But yeah, and adult at that point would have shoved it under the door in an envelope (so it is obvious and doesn't get lost)
As for Chiyo confronting Subaru or whatever - we the audience know the circumstances are sus as hell, and *if* something happened then technically Subaru raped Igrashi, but yeah men can't be raped by women herp derp... Chiyo doesn't know all the sus stuff we do, she has her BF admit to cheating and believes him, she has no reason to dig more. Only an immature bitch would go chasing after Subaru over it when they believe it is legit cheating, so bizarrely we've found an instance of Chiyo being mature? Yeah, broken clocks are right twice a day & all that.