Help with updating to 3.0

I'm not sure how much it's going to help if you're having trouble understanding the PHP errors themselves, but I'll attempt the first couple of items:

Message: Declaration of Siteconfig:: save() should be compatible with that of MY_Model:: save()

This shouldn't have anything to do with the upgrade from CI 2 to 3. This implies that you changed something in MY_Model's save() method (like the number/type of arguments) without making the equivalent changes in Siteconfig's save() method.

Message: Undefined property: CI_DB_mysqli_driver:: $ar_orderby
Filename: core/MY_Model.php

In this case, your MY_Model is referencing a private property in the mysqli driver for Active Record. Since Active Record has changed to Query Builder, the name of the property also changed. Since the property was private and the visibility indicators have been added to the code, it is no longer accessible. You're going to have to rewrite your MY_Model so that it no longer depends on the availability of this property.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide