a) With the newer Xbee ZB or Xbee 2.5 modules you will have a mesh network. If A is not able to talk directly to C then B will relay the message from A to C. So yes, messages are passed on in the network. If node C is an endpoint and a network child of B then I think B will be responsible for passing a broadcast message on to C.
b) With the normal AT command mode you can send a message to either all other modules in the network (broadcast), to the network coordinator or to one specific module. If you want the message to be sent to two other modules you will just have to send it twice with different addresses. In API mode you might be able to send the message to two recipients at once, I am not sure. This mode is more powerful and also mor complex to understand/use.
If A is sending a message addressed to C, and the message goes through B, then B will relay the message without reading the contents. At least that is my impression of how it is supposed to work.
I have not checked the documentation for the specific situation you ask about in your initial post, but I am quite sure that if both A and C try to send a message to B at the same time, then the network will handle it somehow, probably by allowing either A or C to send its message first and for the other sender to resend its message a bit later.
My impression of the Xbee network structure is that it is quite stable, and that you can be certain that your messages will get through to the intended recipient. I am no expert, though. I have a few modules and I have done some testing, and I have browsed the documentation, but I have not used it in any real world application yet.
Regarding Xbee, I would like to pass on a tip I read somewhere. Digi, the company behind the Xbee modules, have a contest where you are supposed to design something using specific Digi hardware. To make it easier for individuals to participate they are selling starter kits
at a greatly reduced (I think) price. Anyone can order these kits, there is no obligation to participate in the contest. I ordered an X4 Starter Kit
which among other things include an Xbee to/from ethernet gateway and a mains powered "wall router", basically a unit that extends the network coverage by acting as a repeater. These kits may not be what you need for robotics, but it is rather cool hardware which is fun to use. Using the X4 gateway (programmable in Python) you could e g access you robot over Internet from anywhere.