I am unsure comment editing is such a good thing. Suppose someone changes their comment so they have the right answer? Other things could come up too. I would suggest putting something editted in later in italics or something else distinguishable but not distracting.

Yeah, if you're just going to append something to a comment, you might as well post another comment. The editing allows us to fix problems with the <pre> tag and the preview box, as we have seen troubles with lately, without having to post a string of comments correcting typos or other small errors.
Someone can't do any more than they could in posting a comment normally; the worst I can see happening is a new user changing their incorrect solution after the official one is posted or something, and even then there is the edit date.

If that worries you, perhaps to make sure that the feature is used only to correct errors, etc, you could only allow problem editing for 12 hours after it's posted or something.

One thing, though, I would suggest that the 'Edited date/time stamp' not be added if the problem has just been submitted, for half an hour or an hour or something, to allow people to fix display problems with their comments and not have it looks as if a solution was fixed later or something (although the actual time/date of the change will show otherwise, I imagine that will be overlooked a lot of the time). I can see using it quite often in place of the preview comment box, since that often doesn't work the way it should, but then a newly posted comment looks as if it was changed after it was posted. That's the only further suggestion I have.