Unclear informations. You say "need a shared datastore" which means a NAS but if the NAS failed you don't have HA on the datastore level. But you also said "protects against datastore failres" so that would require multiple datastore and not one single shared datastore... so : unclear
vSphere HA doesn't automatically migrate VMs rather it restarts VMs on different Hosts
HA doesn't migrate like DRS, it restarts the machines on another host. So it involves downtime.
Unclear informations.
You say "need a shared datastore" which means a NAS but if the NAS failed you don't have HA on the datastore level.
But you also said "protects against datastore failres" so that would require multiple datastore and not one single shared datastore... so : unclear
vmotion and HA are different dude.
Vsphere HA restart the VM on another ESXi host in case of failure