Long story shortened a lot, this is not the first time I've trashed a flag just so I could re-flag a submission with an extended flag reason/description, after reading my original flag reason through after submitting it. Maybe due to wording something incorrectly, maybe due to forgetting HTML formatting doesn't work, maybe due to thinking something is obvious at first.
An "Edit Flag" button could (possibly) be useful to just edit one's own flags as one wishes (except trashed ones), as long as the flag is younger than an hour or something like that (to avoid exploitation of it). Maybe even just a half hour or a quarter. I usually find about two minutes to be enough for me to re-submit a flag with a different flag reason/description.
tom, please allow flag-editing. I just flagged, trashed flag, flagged, trashed flag, and flagged a submission in order to change flag description/categories, and all 3 flags still show up on the Flags page (although two have "Trashed" above them). That's both annoying and cluttering, both for moderators and the submission author.
Being able to actually edit your flag explanation and/or selected flag reason(s) would definitely be very useful. It's little annoying having to trash and then re-flag a submission because you either didn't explain it correctly or chose one or more incorrect flag reasons by mistake.