a) good but also consider a preferred backup ZDC
b) make that two
c) that works
d) perfect
i) NLB is one way and is the cheapest
ii) all XenApp servers are XML Brokers. If you are going to have a dedicated ZDC then most people use that server as the dedicated XML Broker. A dedicated ZDC has no apps installed on it and services no users. It makes the perfect candidate as the primary XML Broker for the WI servers.