Sql-server – Unable to drop database user without login

to relink the login to the correct id you will have to run (I assume that the login is appuser, and the databas user appuser):

alter user appuser with login = appuser ;

From BOL:

To change the name of a user requires ALTER ANY USER on the database. To change the default schema requires ALTER permission on the user. Users can change only their own default schema. Requires CONTROL permission on the database to remap a user to a login

As you explained that you have the db_owner role on the database, it should work.

For a failover-type situation, I use the handy 'sp_help_revlogin' stored procedure as published in http://support.microsoft.com/kb/918992

This procedure, when run on your primary server, will script out each login WITH the SID (this is the important part for making sure your logins are mapped to their respective database users) and hashed password (so the password remains the same on each server.) The resulting script can then be run against your failover server to create the logins. Be sure to review the output before running it.

As-is, it scripts all of the logins in the server, but could be modified to script only the logins you're interested in. It could also be automated to generate a script on a regular basis.