I've been looking into cache again the other day. There is no solution for this. You cannot use joomla's caching with the comment system. In matter of fact you cannot use the Joomla caching with any component that creates dynamic content & lets user interact on the frontend. Also in a matter of fact -> the joomla cache is always turned off when you log into your website. Each component should cache it's own data. A global page cache is fine when you have just articles - but when you have people interacting on the site its impossible to use it.
The progressive caching is in my opinion a good option, but unfortunatly it is broken as written in our documentation. I would love to make this work with joomla's cache, but unfortunately so far I can't find a solution that works other than implementing caching internally for the component.