I did read it but not exactly sure what you're saying? you're confirming the problem after the first 3 points

1. A non registered user create a new thread.
2. A mod edit this posting. --> "edited by mod" is shown
3. A mod changed the category --> "edited by non registered user" is shown

and then you say

4. A mod changed the category AND edit the posting --> "edited by non registered user" is shown

After changing the content of the entry and altering the category again, the entry-is-edited-message says, that it was done by the admin, not by the thread opener.

But point 4 should not even come into it?

The key fact is plain and simple, If I have the settings set for Guests not to be able to edit their posts it doesn't matter whether the moderator edits the content of the message or whether the moderator keeps moving the thread to different categories, it should at no point ever say it was "edited by guest" and it does.