I think originally you did not upgaded the database when you upgrade SMF. You can do this by running the 'large upgrade'
http://download.simplemachines.org/Make sure you follow the details in the upgrade. There might be a shortcut script to just upgrade the database than running that upgrade but I dont want to make a mistake saying something else which might be wrong.
Anyway - it was a mistake changing the prefixes. The 'sp' ones are put in there for SP - Simple Portal data and settings. If you change the names Im sure it will cause errors if unable to find the date in the tables - as you renamed them.
There is no need to change the names - Simple Portal adds its own tables to the smf database - that causes no problems.
How is the situation now?
As you have changed so many things it is hard to say what is best to do. If I get in deep trouble I run a clean install of all files using the large upgrade package
http://download.simplemachines.org/That cleans out any file edits.
Before running the large upgrade you should put back a previous database backup - one not changed by you.
You should also always make a database backup before you change anything - you did that right?
Then you will have a clean site and can start putting mods back in.