How mysqlcluster can be durable and consistent?
Q1:  Data is both sharded and duplicated.  Probably each of your two datanodes has the entire dataset.  If you had, say, 5 nodes, each one could have 2/5 of the data, thereby spreading across 5 machines, yet having 2 copies of everything.  And loss of any one node can be recovered from.

Consistency is handled by a "eventual consistency" model.  If you write conflicting things to different nodes, the system will 'eventually' discover this.  Then some rule (which you established) will kick in to decide which data to keep.