If you’re using a script to do so, make sure it’s handling API limits specifically for “edit” calls. I realized after I tried overwriting mine that it was quietly skipping a bunch of comments, presumably because there is (allegedly) a 1 edit call per 5 second rate limit. Since adding a 5 second delay between each re-write, it seems to be working for me.
I ran into this issue with u/j0be’s Power Delete Suite, I ended up writing my own script to do the job.
Yes, they keep the comment history forever. The idea is that as long as content is not publicly visible nobody will find it via Google and visit Reddit to be exposed to their ads.