I have attempted to isolate a point of failure, and have established that:
changing MAC back to broadcast still functioned,
as did reincorporating the unusual 'XBee.begin(9600);' statement.
This leaves security configuration. I had tried previously to disable this, however I hadn't manually disabled the security features (mode = 0).
I've renabled them, just as they are in creating network, and they seem to be fine also.
My only conclusion from this is either during my use of X-CTU or IDE code, my own or that provided to help me, I misconfigured the ZB modules, seemingly in the security parameters.
I suspect during my use of X-CTU a simple scroll wheel action with the wrong parameter highlighted just prior to a config write could be the cause of these woes.
Thanks for your help - next up... a ZigBee tree topology!