1) As you found, that error code can be attributed to many causes and there is no one 'fix' for any of them.
2) Are you referring to the Restricted Sites in the "IE Internet Properties Security" settings panel, or some third-party program?
2a) If it's some third-party program, have you tried disabling it?
3) No, the ending of support for XP's SP2 just means that you'll have to have SP3 installed because some updates that were associated with files that were part of SP2 have been later updated with SP3 as well as other critical updates released afterwards, etc..
4) You must have an Administrator account on the computer as well as any other normal User accounts that were created. In other words, you can not delete it.
5) NO!!!
6) Reboot into "Safe Mode with Networking" and try accessing Windows Update.