How to create Nested Accordion in Drupal 7?
Drupal View Accordion triggers the accordion for the innermost Group Header. Though Nested Grouping can be done but the accordion effect will only applied to inner group. 

For this we have to apply our own JS. I have created a module in which current accordion plugin has been extended, which itself was extending view plugin style. And additional setting has been added to it for nested accordion.

I have committed a [D7] Views Nested Accordion module on sandbox for this.