But I've read somewhere that KV cache for speech-to-speech model explodes in size with each turn which could make on-device full-duplex S2S unusable except for quick chats.
if you don't have a contract with the utility you almost certainly violate the law, at least in Europe, but then again I don't know what the US regulation is
That's a contract between users and HN. Airtrain is a 3rd-party.
If HN API exposes personal information publicly through their API then there is a problem.
And AFAICT the only way for HN to prevent user comments from being used by 3rd-party is preventing access to those comments, meaning a) sign-up will have to be more stringent and b) visitors will have to sign-in just to read (or scrape) comments.