Can I comment on Beehaw (or lemmyNSFW, but I’m at work so not going there right now) using this account I have at lemmy.world?

Beehaw, no. They defederated from Lemmy.world. More information is at lemmy.world/post/145337 (iffy formatting) and beehaw.org/post/567170 (easier to read)

LemmyNSFW, yes. The answer is yes for most instances.