about the eyebrow phrase; you mentioned that this is translated from a Spanish translation so it's hard to say through the middle layer (I don't really know Spanish and didn't catch anything from a cursory google search), but I believe there is a phrase in Japanese which is something like "to be able to read eyebrows", which could possibly be the source. I believe it means someone is like an open book, kind of like how Rintaro is at this point relaxed and honestly himself more than he used to be.
But again, I couldn't find the raws of this chapter either with my cursory search, so couldn't check. If anyone does know, please do share, would be nice to know how it actually is