"I tried to create a group hierarchy in a way that I can pass the responsibility of argument validation to the argument parser." 

I looked at your example in:

https://bugs.python.org/issue46058

The many levels of nesting groups was confusing, but now I realize that by nesting argument_groups in a mutually_exlusive_group you were try to implement some sort of any/all group within the xor group.  

This cannot be done within argparse.  Mutually_exclusive only implements a flat xor.  Argument_groups are used for help display, but play no role in parsing.

Some years ago I explored the use of nest parsing groups:

https://bugs.python.org/issue11588
Add "necessarily inclusive" groups to argparse

I was trying to allow for nested groups that implemented all logical options - xor, or, and, not.  Defining such groups wasn't hard.  And I think I got the testing logic working right.  But usage display required too big of a change to the formatter, and left too many loose ends.  So I have let that languish.

Now I recommend focusing on doing the testing after parsing.  Use meaningful defaults where possible, and use `is None` to test whether a users has provided a value or not.