There are a few active threads on this already. Please contribute your thoughts there instead of making a new one as we only allow one active thread per topic.
This is brought up time and time again, and the general opinion for people that have been on this site for a year or more is that no, that would just be abused or take away some honesty.
I can agree there, but I do think it would be nice if we could append to our comments, or edit them as long as the original comment could be viewed still. Most people just reply to our own comments with any corrections, but that often goes unseen by the person we were talking to.
No, and I don't like your signature. Its bias against monotheistic beliefs and doesn't specify what god. I could for example believe in the Golden Goddesses of Hyrule.
Not a good idea considering how many trolls and such there are on this site. If we had the ability to edit comments people are going to abuse the system and post a nasty comment on someone's page, then edit it before they get punished so they get away with it.