Downvotes are definitely a disagree button and anyone denying it is just coping or lying about their own usage of it. Reddiquette was the worst thing that ever happened to internet culture
Almost anything else or nonexistent. Maybe a way of filtering lazy, unhelpful, or uncivil comments. Downvotes being a disagree button just seems blatantly toxic. Expressing disagreements calls for using actual words.
Not sure about that last bit. Do you know how many people are wrong on the Internet? Lemmy isn't my job, yo: I can't engage in a dialogue with with them all.