There is a depth limit of 12, a technical limitation, but that just means you cannot reply to the deepest post; return to a post elsewhere (typically 'higher' or previous) in the thread and reply to it.
As to your other questions:
-> Members cannot edit their own posts, for reasons outlined in this forum's sticky (READ ME).
-> The PM functionality is currently under construction, as noted in this forum's sticky.
I was the 21'st person to post on a thread and now my question cant be answered. It's very important as it involves a getting a computer mobo fixed. Can I edit my message to include contact info? How can I contact a member who posted? Why is there the stupid 21 posts rule?