Okay, so now you are at 2.5.1. I still wonder if you have been hacked or not.
Do me a favor: go to your Users tab in your Dashboard. In the “Search Users” box on the right, type in “WordPress” (no quotes). What message do you get?
Specifically, we’re looking for a message with a red line around it that says:
No matching users were found!
If you DO get this message, then you have NO phantom users and that is good. Proceed to Step 2:
STEP 2.
Do me another favor: GO TO YOUR PLUGINS DIRECTORY AND REMOVE THAT INDEX.PHP. It’s written wrong, or something, and it’s messing things up in there. Earlier in this thread you said you had removed it, but it is still there.
Once you remove it, try editing/deleting posts. Can you do it now?
If not, deactivate all your plugins. I know you said you’ve done this already, but you have since upgraded and I’d like you to try it again.
Once you’ve deactivated your plugins, try editing/deleting your posts. Does it work?
If it does, it is a plugin problem. You need reactivate one plugin at a time and test your page editing/deleting ability each time.
If it STILL does not work, then maybe somebody else can jump in and help out, because I am fresh out of ideas.