How can we deploy Keyclock redundantly on Azure

That’s what the HA clustering is, yes.
Difference between domain and standalone mode ist just the management of the resources like DB connection and so on via a centralized domain “controller” server (management instance), which isn’t available in standalone mode (you’ll have to configure every instance individualy). But, as I already said, if everything is automated, you won’t need domain mode.
For further info, I suggest to read the wildfly docs on these topics.