Your questions 1 & 3 are similar.
1&3 : It is little cumbersome to handle this case but nevertheless it is supported. When configuring DCs in XDR section, you can use the public-ips of the remote DCs. However, you need to add a mapping of internal-ip to external-ip using the ‘dc-int-ext-ipmap’ config option. Make sure you give the mapping for all the nodes of remote DCs. For details of why and how to do this, refer to http://www.aerospike.com/docs/operations/configure/cross-datacenter/network/.
2 : I dont think you can save the cost as the amount of data transfer is the same in star topology or chain topology (AWS charges by the amount of cross-region data transfer). You can save on the latencies by seeing the latencies across amazon regions and choosing your topology. One thing to note in amazon setup is that the latencies are not highly consistent. In AWS setup, we have seen scenarios where the XDR transfers are stalled for a bit and then resume. XDR is designed to handle this scenarios, so it will eventually catchup.