using multiple database on same application and issue with URI Routing

3) I would store it in a separate table if this is in your design ok, so its easyier to use.
4) You should go with CI3 and create a subfolder in your controller directory

PHP Code:

/controllers/ibm/User.php
/controllers/microsoft/User.php
... 


You can access it then by http://www.example.com/ibm/user/login etc.