Your comments

For the moment, you can use view.window(), which will return None if the view is not current visible
For what it's worth, Linux is the only platform where this doesn't happen yet
the flag sublime.LITERAL should do this
In build 2032, snippets will no longer trigger on tab when a snippet is currently active. They can still be triggered explicitly by using ctrl+space.
Build 2032 has HTML completions
File loading itself has always been async, but there was still some IO done in the main thread. Build 2032 resolves this.