About Reimu and youkai, it's just her job to beat them up if they do something dangerous. However, she doesn't have any malice towards them, so she doesn't mind them as long as they act nice. For the most part, she just doesn't care about them, but she can actually be friendly too.
Indeed, if anything I would call Reimu (and Marisa to an extent) indifferent to most youkai. She doesn't even seem to mind being around them too much. With how Gensokyo is set up there's no real need for Reimu to go out actively hunting for youkai to be exterminated, only if they are causing problems. The youkai live their lives and the humans live theirs. They do business and whatnot. Someone causes problems (human, youkai, god, or otherwise) they are dealt with by someone and almost always forgiven.
As for Reimu and Marisa's friendship, I would definitely call them really close/best friends. Sisters in incident resolvers if you will. There's no reason Marisa couldn't visit the shrine whenever she pleases and just hang out with Reimu for a bit or even have a quick duel. They always seem to be hanging out in the official mangas. They really haven't been "enemies" since SoEW (Touhou 2) and even then Marisa wasn't really malicious toward Reimu. The IN fight between them was definitely a misunderstanding and they didn't really dwell on it or hold a grudge against each other.
What I find interesting is how different casts would interact with each other and the world. Like, how often does the UFO and TD cast butt heads? Do SA characters ever go skipping around the surface? (I know Satori wouldn't and Koishi already does so...) What the hell do IN characters do all day? Just things like that.