The channel_id is the channel_id of the page that you are on.
The broadcaster_id of channel_id is one and the same
When the broadcaster is using your extension the user_id is in the JWT is that of the broadcaster.
You can use the channel_id from the JWT to lookup the channel in the API
To answer your Points 1/2 better. You can take the channel_id and perform that lookup against the Twitch Users API. It can’t be done via the helper itself.