I think you had a reply confirming you need to import the captive portal content onto the local so it looks the same. That's cool.
Something else worth considering as you mentioned resilience:
If your master fails and you're using the local database to authenticate guests, your local won't be able to do this. This is because the local looks to the master for this authentication (which is centralised on the master). i.e. this database is not copied into the local controller. If your master does fail, the easiest temporary fix is to copy the local user database (assuming you have a backup) into the local manually, and then enable the local controller database check.
"aaa authentication-server internal use-local-switch" (from config mode on the CLI).
Either that, or buy a dedicated backup-master controller!