As I mentioned in the part 1 post, I managed to get things working, but the Airport Express (AEX) was still connected to the Ethernet. As soon as the ethernet connection is gone, the Airport Express and my airport on my MacPro are no longer getting a DHCP lease. Giving the AEX a fixed IP does not solve this (though the amber blinking light turns green).
So the people who told us that it would not work are right!