I only see screenshot#5 attached.
To answer your questions:
1) Mostly relevant when you have parent and Child policies. Mandatory rules are looked through first, then any child policies, and finally default policies. I think of Mandatory rules as something all traffic has to check first before child polices, while default are generic policies you want to apply for any traffic that does not match Mandatory or Child Policies.
2) I don't know of any easy way to do this other than looking through the policy section and checking under the "Logging" tab.
Ideally, the Network Discovery rule includes host discovery for all the LAN segments on your network. There may be cases where you can exclude load-balancer's and other high volume devices from this.
According to the Firepower guide, one of the reasons for having Host discovery is:
Alerting you by email, SNMP trap, or syslog when the system generates either an intrusion event with a specific impact flag, or a specific type of discovery event
https://www.cisco.com/c/en/us/td/docs/security/firepower/60/configuration/guide/fpmc-config-guide-v60/Introduction_to_Network_Discovery.html#concept_B9C9F7BF250847D6A4FB888CB738EA17
