Discussion board is the second tab on any presentation page, beside Details. Inside it is a single card, headed Live thread, with a box to write in.
The small word beside the tab name is the state of the board. Read it before you look for the box.

A board opens the day before the conference and never closes again. The abstract and the presentation details stay visible throughout; it is the conversation and the media that belong to registered delegates.

Write in the box, add an emoji if you want one, and Send. Your post carries your name and links to your profile — the same profile that holds your own presentations, so people can follow you back to your work. Replies go one level deep: a board is a single conversation in order, not a tree.
The board outlives the session. It stays open after the conference ends, alongside the media — a question asked three weeks later still reaches the presenter.

A discussion board is not a comments section. What gets a real answer looks much like what works in the room — specific, generous, and answerable in a paragraph.

Check the word beside the tab. Upcoming means the board has not opened yet — boards open the day before the conference. A padlock means you are not registered for this conference.
No, and the naming is confusing. This board belongs to one presentation, and the card inside it happens to be headed Live thread. The conference’s own Live thread is a separate tab on the event, for the whole conference.
Yes, one level deep. A reply to a reply is not available — the thread stays a single conversation in order.
Two ways at once: a notification in the bell, and an email. The comment count on your My events card also moves.
Yes. The board stays open alongside the media, which is the point of keeping presentation pages up.
Nothing expires. The presenter has been notified twice over, and the board stays open — an answer next week reaches you the same way.