(I saw your other post, but figured i'll comment here as well. )
Missing from the GUI?
and you are trying to configure Unified Manager, not System Manager, correct? (9.9.1P8 sounds like you're in System Manager. )
Augment is probably the better word. Unified Manager allows for things like specific performance and capacity alerting. It does the most common EMS items as well as customization in what you want to monitor. It also allows for a single point to monitor EMS messages for Clusters in an environment.
Depends. But most that I've set up for customers and seen in the wild is a mix of both.
Systems get setup to alert on Error/critical. either email or trap.
UM will get set up with perf alerts, snapmirror reports, capacity reports/alerts etc.
Separate. Though UM will monitor EMS from clusters its connected too.
EMS comes from the EMS system on the ONTAP box itself. Anything from here basically - https://docs.netapp.com/us-en/ontap-ems-9111/pdfs/fullsite-sidebar/ONTAP_9_11_1_EMS_Reference.pdf
Unified Manager has a lot more.