Hi,
Sorry for such a delayed response - I'm not quite a regular visitor of these boards as i should be. Try disabling the billing system from cast-control, you will still be able to use the whmcs plugin without difficulties but this will remove all references from cast-control to the cast-control billing system.
You might also need to remove the <---Begin Billing---> and <---End Billing---> tags from the template or your plans link will not display.
Let me know if you still have any difficulties.
Regarding the username and password text, i also think it might be more appropriate to use the full wording and will make this change for future versions.