1. Not sure what you mean. How can you force someone to respond to something? Do you have examples of this in other applications?
2. Simply comment on the thread to push it up in the feed and make sure to tag them, they will then see the thread in their inbox. Advice them to keep the email notification enabled for changes in their Yammer inbox if they're not used to using Yammer.
3. There are webparts to integrate Yammer in SPO. In modern sites, just search for the available Yammer webparts.
4. The permissions are managed by the group admins. They can add others, make them admins, revoke their admin permissions. Not sure what you mean with "D&G RFP". There is no easy way to prevent people from creating silos. You have the option to allow a private group to be shown in search (list in directory in group settings). This is also always enabled for connected groups. Just make sure that the users know that they first have to search before creating a group. There is also a "related groups" feature that you can use. They can then easily see that a related group already exists.