Not sure about Monaco (yes I remember that I've see photos of Albert and Charlene bow/curtsy to QEII too) but I still believe that GD of Luxembourg doesn't need to bow/curtsy to Majesty.
0:06 Henri didn't bow (and unfortunately it didn't show MT)
1:33 both Henri and MT didn't bow/curtsy to Beatrix (but interestingly Guillaume didn't too)
0:25 both Jean and JC didn't bow/curtsy to QEII (and again, Margaret and Anne didn't curtsy to GD couple too)
I think we will never get a clear answer because somehow I think it really depends on individuals and they don't have an actual rule about that