How quickly you now that the system is down depends on the schedule you run heartbeat. So if you ping it every 5s, in the worst case you know it after 5s + ingest time from beats to ES.
For your second question: I think you need to share the resulting events from heartbeat to give further feedback here.
For alerts: Have a look here: https://www.elastic.co/guide/en/x-pack/current/watcher-getting-started.html