Your patch is incomplete, without documentation or tests.

Your example is incomplete.  That is _CustomAction?  What namespace does your patch produce.

It's been a while since I worked on the intermixed patch, but as I recall the OP was happy with the fix.  Also I don't recall any talk about 'matching "optionals" with "positionals" with they relate to. Was that part of the discussion?

On Stackoverflow I have seen questions about pairing arguments, and answered some.  I don't recall the best answers.  The tough version is when some of the grouped inputs may be missing 

In your example I'd suggest making '--tracks' a nargs=2 and 'append' option.  'choices' would have to be replaced with a either a custom 'type', a custom Action, or post-parsing testing.

I haven't had time yet to test your patch.

One question I frequently ask posters who want to parse complex inputs is: 'how are going to explain this to your users?'  'What kind of usage and help do you want see?'