Making posted comments read-only is quite appropriate. Sometimes post-submit, you see something or learn something that you would want to alter within your comment. Once submitted, put a bold green or contrasting color line that states, "You have :15 to edit your comment".
Making comments read-only immediately, forces one to either ignore or post a follow-up comment and artificially raise the comment tally.
Responding to comments and a brief discussion between Bruce and myself:
N.B.: This is similar to the LinkedIn functionality for group discussion posts and works quite well.
N.B.: The edit window is arbitrary and may work better at :05.