Send an Email from CiviCase
Yes and no. You can add a plain reminder to send an email to the timeline, but to send the actual email they'd need to expand the roles section and click the mail icon.

Or they can send a regular email from their mail program and use the email processor feature, which will autofile inbound emails on the case if the subject has the right code in it. See http://book.civicrm.org/user/current/case-management/setup/ down where it says "filing emails on cases".