You can use either of the approaches you identified  By using performBeforeActions of your custom comment social component operation extension and insert a relationship between current_user_id (as start node) and subscribed resource node (as end node) as per your required type (notification/email-su...

Sandeep,What Janice explained is true for all production usages of AEM communities as an independent module.That specific licensing requirement is for those AEM customers who have licenses for other modules (but not of AEM communities) running in production setup but want to extend their Sites/Asset...