RE: Search articles from case's activity pane
Your screenshot is of 'Article', you access this by going to service>>articles
While under social pane, you can only see 'knowledge Articles'. These are part of 'Interactive Service Hub'
Go to settings>>Interactive Service Hub>>Knowledge Articles
If you have not yet started using it, here are few options:
1.Convert your articles to Knowledge Articles (Export Import)
community.dynamics.com/.../216544
2. Export a template for Knowledge Articles>> fill data and import it to CRM, to be able to see it under cases
3. Go to advanced find>>Look for -Knowledge Articles>>create new, and you will be able to see them under cases
Basically, what you see on emails are Articles and what you see in KB records tab of cases are knowledge articles
If you want to use 'Knowledge articles' on emails as well, create a custom button, which directs you to Knowledge articles