Resolution:
The synchronous and asynchronous only pertain to message stores and their metadata. The failsafe property on a destination dictates which is used. Failsafe will use synchronous store.
System wide asynchronous mode is not required because it is the default. If there are no entries specifying failsafe property in topics/queues configuration files then all messages for all topics/queues are stored asynchronously.

Adding the entry '> failsafe' in both topics and queues configuration files will make all topics (i.e. messages for durable subscribers on all topics) and queues to be stored using synchronous mode but it will also enable dynamic creation of all topics and queues. If dynamic creation is not a problem it can be used as a way to make all topics/queues to be failsafe (synchronous). Otherwise it has to be managed on a per-destination basis. Notice that if '> failsafe' is defined, there is no way to disable failsafe for a particular topic or a queue.

Is there a way to specify system wide asynchronous or synchronous disk writes in the configuration files?