Hi Derek,
yes, if the MAC dest was 0x0000, the MAC ack could be obtained from the corresponding node only (i.e. the coordinator). However, as I wrote in my previous post, it seems the coordinator doesn't see the router 0x7B40 at the network level (the corresponding 'in' and 'out' costs are equal to 0). At the same time the router 0x7B40 sees the coordinator as the 'in' cost is equal to 1.
Regard,
Ilya