Alright, so here's the first real issue. Here's what I did on GitHub:

I also edited a comment and subscribed/unsubscribed in this sequence.

Here's what the GitHub Events API gives me for that:

That is, I only get the major events (comment, close, reopen).

There's also a separate "Issue Events API", which I haven't looked at yet. So it's possible we can just poll both APIs, which is messy but not especially problematic. I'll see how bad Pull Request-related events look first.

The "Issue Events API" appears to provide the remaining events in a different format with different rules. It does not provide any duplicate events, so, e.g., one API retrieves issues being created and commented on while a different API retrieves labels, milestones, etc., except that both report opens and closes, I think. Anyway, seems mostly workable.

Will this be a one way or two way bridge?

Two-way.

Where does this feature stand? I wasn't able to find any documentation on how to use this, but it also looks that a non-trivial amount of work has been completed towards this goal. Is this something still in the immediate roadmap? Being able to easily integrate GitHub issues into Phabricator (and back) is a big roadblock to adoption for us.

See Planning for help with timelines and status.

Hard to say since it fell off the roadmap a year ago, but we do have rough plans to ship Nuance (pre-requisite) in the shorter term.