I have compared current behavior with you explanation and I agree on the enhancement.  'parens' seems more useful than 'default'.

'default' is a misnomer for current IDLE since the current default in config-extensions.def is 'expression', not 'default', I am inclined to add synonym such as 'opener'.  Do you have another suggestion?

On the todo list, the first item is covered by the newish Extensions tab, and should be removed.  What the tab lacks is input validation.  Immediately, pressing [Help] when on the Extensions tab should display text than included the parenmatch styles and the meaning of delay 0.  I will add this.

The second item mentions other Emacs stuff.  Just curious, do you know if 'parens' is part of this?  The 'below' just mentions the 'highlight when cursor returns issue'.