I can open the file directly in sublime, but when I try
I get an error ...
Have you tried escaping all "\" like so "\"?
I think that might help.
Also, you can do view.runCommand('openBrowsableProject', "path/to/project"]). That's probably the preferred way to call runCommand.
I've just noticed that you aren't using the standard .sublime-project extension. Is there a reason for that? Sublime seems to be able to read both, at least from Project > Open project, though.