Hi,

1. If I purchase the embedded addon for wpDiscuz, will it also work for wpForo?

 We're really sorry, but the addons designed for the wpDiscuz plugin are specifically tailored for use with wpDiscuz. Unfortunately, they will not be compatible with the wpForo plugin.

Is it possible to share Instagram, Twitter, and Facebook posts within wpDiscuz and wpForo?

You can only embed those using the corresponding addons. For wpDiscuz, use the wpDiscuz Embeds addon, and for wpForo, use the wpForo Embeds addon.

Will adding addons later on affect previous posts?

Please rephrase your question. 

Is there an addon available for enabling private messaging between users?

In this case, you can check out the wpForo Private Message addon: https://gvectors.com/product/wpforo-private-messages/

Can old images and videos be deleted from wpDiscuz and wpForo?,

This is the wpDiscuz plugin support forum so we can help you only with the wpDiscuz plugin. By Default all wpDiscuz image attachments are inserted in the WordPress Media library, therefore you can see and manage those in the Dashboard > Media admin page. 

Is it possible to add blacklist words or report comments in wpDiscuz and wpForo?

Please read this doc: https://wpdiscuz.com/docs/wpdiscuz-7/wordpress-settings/comment-moderation/