What part of speech are 应答用语?
I'm not an linguist so here's only my personal opinion.
According to a modern classification described in Wikipedia, words for a short answer should belong to interjections in English. Note that interjections in English are further divided into primary and secondary interjections, with the former serve only as interjections (e.g. "Yes", "Hoorah!"), and the latter may be consist of one or more words that serve as another part of speech in other contexts (e.g. "No", "Bloody hell!"). The same criteria is also valid for Chinese ("哎呀!""呜呜呜……"vs. "我的天!""不!").
The definition of interjection in Chinese is: "Interjections are words that express various emotions. They have no grammatical connections to the context. " Therefore, all the words or phrases you listed as 应答用语 are undoubtedly interjections.