Regarding your first question:
Is there anyway to show the equivalent of deployment regions / data-centres in the deployment diagram?
I generally use Notes for that.
And your second question:
What about Platform as a Service? The whole Machine Instance is a Node idea completely breaks down at that point. What on earth do you do in that case? Treat the entire PaaS provider as a single node and forget about the details?
I would say, yes for your last question. And I suppose you could take more details from the definition of the deployment model and its elements. Specially at the end of this paragraph:
They [Nodes] can be nested and can be connected into systems of arbitrary complexity using communication paths. Typically, Nodes represent either hardware devices or software execution environments.
and
ExecutionEnvironments represent standard software systems that application components may require at execution time.