Maybe you can try it out, but have your first round of instructor users create local passwords. Then set the ldap failover to only failover for users not in the ldap database. If there are no problems, everyone gets authenticated through ldap. If there is a problem, you can shut down the faculty app server and your faculty can still authenticate into the student server.
Regarding the second issue - any user with "professor" permissions in the admin course is a webwork admin. So, you can add your ldap users as professors to the admin course and then delete the "admin" account.
Hope this helps,