The MID Server's user account must have the mid_server role to interact with a ServiceNow instance. This role allows the MID Server to access protected tables and perform discovery and orchestration tasks on behalf of the instance. The other roles are not related to the MID Server functionality. The discovery_admin role is for configuring and managing discovery, the sm_mid role is for using the Service Mapping MID Server, and the mid_discovery role is for running discovery probes and sensors.
:
Setting up MID Server user and role
Configure Windows MID Server service credentials
Correcting MID Server Windows service account user and permissions

A scheduled job is a background process that runs at a specified time or interval to perform a specific task1.
It is not a method for deleting specific CIs that are not discovered in 30 days. The other options are methods for deleting or updating CIs based on discovery data. A UI policy is a script that can dynamically change the behavior of a form or list2. A service mapping is a process that creates a map of the relationships between CIs that support a business service3. A data policy is a rule that enforces data consistency and accuracy by validating the data entered into a record.
1: Scheduled Jobs 2: UI Policies 3: Service Mapping : Data Policies

A horizontal discovery pattern is a sequence of operations that collects information about a specific type of configuration item (CI), such as an operating system, a database, or a web server1. A horizontal discovery pattern does not use the connectivity section, which is only used for the application pattern type2. The connectivity section defines how to connect to a CI and run commands on it3. A horizontal discovery pattern uses the following sections1:
* Variables: Defines the variables that store the data collected by the pattern and map them to the target table fields.
* Identification: Defines the criteria to identify a CI and update or create a record in the CMDB.
* Operations: Defines the actions to perform on the target CI, such as running commands, querying databases, or parsing files.
1: ServiceNow Docs - Horizontal discovery patterns 2: ServiceNow Docs - Application pattern types 3:
ServiceNow Docs - Connectivity section