Latency Threshold – I would say to start with 50-60 Seconds to avoid any false positives.
Failure Counter threshold – I would suggest to start with 5-6 for the same reasons.
Admin Notifications – I would suggest to notify via SMS as well, in case email is down. But if you are doing that, you will firstly have to make sure you are syncing the Mobile numbers from AD using the correct AD Attribute in Mimecast(https://community.mimecast.com/docs/DOC-1478). Then you also have to make sure that you set that attribute up to be used as the mobile attribute for SMS notifications in Administration > Account > Account Settings > System Notification Options. Then, you also have to make sure that the admins that need these SMS notifications, subscribe to them. Guidance on subscribing here – https://community.mimecast.com/docs/DOC-2085. Further details on using SMS in Continuity Here – https://community.mimecast.com/docs/DOC-2104.
Affected Group – You have to select a group that covers all users. Otherwise when you get the notification, if you opt to start a Continuity Event from the dashboard, it will only affect the Admin group you have selected.