While I continue to follow argparse issues, I'm not doing development (with my own repository, etc).  

I haven't revisited the issue since 2013, and don't know that much more about Python file handling.  Occasionally 'FileType' issues come up on StackOverflow, and my most common suggestion is accept a string, and open the file yourself.

In writing a solution, keep backward compatibility foremost in mind.  We don't want to mess things up for existing code.  And keep FileType simple, consistent with Steven's original intent - as a tool for simple input/output scripts.

As for the idea of 

2 - Making the argument namespace itself support context management

someone could experiment with a new Namespace class with the proper enter/exit methods.  The use of a custom Namespace class is documented 

https://docs.python.org/3/library/argparse.html#the-namespace-object

So the idea can be implemented and thoroughly tested without altering the default Namespace class and its use.  (With one caution, subparsers won't use this custom class.)