> Use case 1:- In dev , do we need to give the template authors access only to developers .What about in QA - is it only to QA folks? Subsequently in stage & prod(only for business authors) ?
It depends. If developers need to create/modify/delete templates in Dev to do their smoke-testing/cyclone-testing, then yes, you'd need to grant them access. If you want to FORCE definition of templates in Git (ie. define templates on local dev AEM, and commit to Git), then arguably developers shouldn't have write access to /conf/.../wcm/templates and friends, since that opens the door for them to make "tweaks" at Dev run-time and forget to sync them back to Git, resulting in a delta between Git and Dev. Exact same for QA, just "does QA need to make changes to perform their QA?"
> Use case 2:- Suppose if a person is having access to template authors group, does he/she really needs any predefined access to other folders?
At least on Dev/QA, a Template Author would probably need Content Author access to create pages using the templates. On Production, this would be up to the business based on how they want to divide responsibility between users/teams.