Cannot edit links (or source code directly) in Scheduled Reminders
OK, I'm going to be bold and answer my own question. According to https://civicrm.org/extensions/tinymce, starting with CiviCRM 4.7, an internal editor CKEditor was provided to handle all Civi backend editing. Since we started using Civi prior to that, we had the TinyMCE extension installed and I should have been aware to disable TinyMCE in favor of the default CKEditor at that time. 

I'm interested if anyone else has anything to add. At the very least this post does note an evident TinyMCE bug with our Wordpress/CiviCRM versions configuration.