Hi Yogi,
Provisioning Directory is a mandatory component because it contains all information related to provisioning.
So not only Global Users but also all information needed to connect to a target system (i.e. connect to this Active Directory's domain with this specific administrative credential).
When you run an explore you load into Provisioning Directory all accounts that belong to a specific target system.
Regarding point 1, as for now CA suggest to not use Provisioning Directory as user store.
Regarding point 2, it seems that you are missing basic CA Directory concepts related to HA (high Availability). CA Education as good starting course to acquire these concepts.
Hope it helps,
ciao, Roberto