I can't imagine there isn't a simple strategy to solve this issue of stale nfs handles preventing a machine from shutting down normally, but I'm not sure what that would be. It's pretty annoying to have to force a reboot just because a wifi connection dropped out at some point while watching endless 'waiting for nfsmount' messages. 
how to prevent reboot being stopped by nfsmount?
mount;nfs;fstab