I think a “Reply via Chat” or “Open Topic Chat” below the post would be very helpful. I didn’t find it intuitive to click on the chat icon next to the topic title.
My biggest challenge is figuring out what to put as a reply to the topic versus what to do with chat. Perhaps it’ll be up to the poster to clearly spell out which feature is used for what and perhaps I’ll find it easier to wrap my head around this when I test this with an actual use-case.