All right, it turns out the issue was on my end (what a surprise… Twitch doesn’t ban the word Hello after all!)
Thanks for your help. I wasn’t receiving NOTICEs because some combination of “CAP REQ :twitch.tv/membership”, “CAP REQ :twitch.tv/tags”, and “CAP REQ :twitch.tv/commands” had to be sent to the server first. I think I would have long given up on this if not for your responses, honestly, so thanks! Everything’s sorted out now, thank goodness.
And, if you’re wondering, I really, really hate to admit it, but… apparently the word “hello” was on my channel’s banlist. From several years ago, I’m guessing. Sigh.