If you did not find the question you were looking for here, throw u/consulnappy a private message on Reddit, and along with a response, the question may be added to this page.
No. RedditMetis does not store any of your comment information. It only stores the result for a few hours to prevent spamming of requests.
Delete your comments on Reddit so that RedditMetis would not be able to see them. Be sure to wait a couple of hours before refreshing though, because RedditMetis remembers the most recent results of a profile scan for a couple of hours before forgetting them.
No. Casing does not matter.
First thing to check would be the spelling of the username. Make sure you did not prepend the u/- prefix on usernames. If it still doesn't work, you may have been shadowbanned from Reddit.
If the issue persists when searching for any user, there might be plugins/content blockers that prevents RedditMetis from reaching Reddit.
As an anti-spam measure, user queries are cached for a (hidden) amount of hours. So you have to wait a couple of hours before RedditMetis starts processing your new comments again.
To prevent server timeouts and overload, the retrieving of submissions and comments have been made client-side. It is your machine that connects to the Reddit API. Thus, the speed of data gathering is dependent on your network speed. In addition, the Reddit API rules state that no more than 30 requests per minute should be made, thus RedditMetis cannot retrieve all the data at once.
Metis, in ancient Greek religion, was a mythical Titaness which, by the 5th century BC, was regarded as the mother of wisdom and deep thought. You can probably see how this matches RedditMetis' data scraping capability. Coincidentally, Metis is the name of an asteroid first observed in 1848.