It appears everywhere regardless of if the user is in a group or not, they appear underneath the edit activity as "summary: editing page" or in the recent changes and user contributions in (example summary) format
Comments for actions like moving pages and promoting users can be viewed using Special:Log. Comments made for uploading images appear in the "File History" tab of the file's page. If uploading a new image (as opposed to a new version of an existing image), the comment is also used as initial content for the "About" tab; I think.
I have made mistakes myself. Unfortunately, edit summaries cannot be altered once submitted. They can be hidden but that ability is reserved for staff and they won't use it for something as minor as a typo.