i agree! i don't think butters is just his lackey. it certainly started like that but their relationship has definitely evolved over time. there will always be a power dynamics component i think, mainly from cartman's side (however we saw him pleading to butters in s20 to stop being so mean about heidi lol) but i think there's more camaraderie between them than in the early days where cartman wanted to humiliate butters or used fear to make him obey. it might also be because cartman has changed and butters too, he's definitely less innocent than back then.
and yeah omg i can see it, i definitely think they text a lot and butters would be the first person cartman told about his crush on kyle. i'm 100% convinced of it. if there's anyone that knows cartman's dirty little secrets it's butters because, for some reason, his well-meaning nature tends to make cartman believe that his confidences are safe. also because probably no one would believe butters since they consider him an idiot haha. i can also imagine them having sleepovers often where they gossip like little idiots over pizza and marathon shitty movies
while butters has been known to side against cartman, i DO believe that cartman himself doesn't see butters as a danger. in most cases anyway. kyle even went to cartman in wieners out to essentially ask him to talk to butters so the idea that cartman is the one that can subdue butters is a general perception. butters is definitely a wildcard though but the important part here is that cartman finds him mostly harmless so yeah i'd agree he feels safe with him
i love buttman too...i think they both kind of have a crush on each other they don't really acknowledge...