Interesting - The MEM-AMS availability is there (I used July 10,11 and 12 to search, and there are standard award flights all three days. Also, AMS-ATH has seats. Its just that there are no seats available from any of the four airports you mentioned to MEM or DTW.

Try this website (source website for the WPAA site which no longer works: http://www.nwa.com/jp/en/home.shtml

Put in each segment of what you want (PIB-MEM or DTW; MEM or DTW - AMS; AMS-ATH. Try a variety of dates: for example.

I'd try calling NWA if you can't find any agreeable flights to MEM or DTW, cause the transatlantic availability is there.