To use the 'config ap user....' command, the AP needs to be joined to the wlc. If your issue is getting the AP to join then this will be no good; because you did not set the username and password when the LAP joined the WLC, you have removed any config privellages.
You can either reload the software image on the LAP with a tftp server or create a means for the AP to dynamically find the WLC, via DHCP opt43 or DNS etc.
"clear lwapp ap controller ip address" and "lwapp ap controller ip address" commands work only while running a recovery image that you use to convert from Autonomous to LAP. Once the LAP joins the WLC, it will download the new LWAPP image and overwrite the recovery image. LWAPP image doesn't allow you to use those commands.
You have two options:
1. Convert it back to Autonomous, then convert back to LAP using recovery image. You can then prime it with the controller IP address with those commands. Converting back to autonomous is a very quick and easy procedure