I am a bit uncertain what's your goal. Do you want to manage your cluster, e.g. monitor your cloud's health, or do you want to create something like Horizon? Your first pararagraph indicates the former. The third question and your question about tenants in the second question indicates the latter.

If you want to create a new dashboard, get your inspiration from Horizon. It connects to the OpenStack services by issuing API requests. If you want to create a tool to monitor your cloud, why don't you use existing solutions, for example based on Nagios? In any case, you need something that checks whether all processes are running, all files are in place with correct permissions etc. This is normally accomplished by running agents on the hosts.

The web site users should be the same as the OpenStack users. Horizon doesn't keep a separate user database as far as I know. Just use Keystone. If you use the web site for managing the cloud, you only need one user, namely admin, I would think. The MariaDB cluster keeps the permanent state of your cloud resources. I would keep temporary data that changes often in a different place to avoid stressing the DB too much. OpenStack makes use of memcached, for example, or MongoDB for metric data.

I am not sure if I understand question 3. In any case, instances don't have a time limit, but it's easy to delete them automatically using cron or at or a more sophisticated scheduler.