I agree with you on Ruby. I have no idea why Cinder went after the Fall Maiden instead of the Summer one. However, I think Cinder was probably next in line to receive a Maiden's power, but was denied for some reason. This caused her to seek an alliance with Salem (power in exchange for loyalty). The glove she used to absorb Amber's powers had the same symbol that Salem's cape had on the back of it. Also, if Cinder was already a Maiden, why would she need Emerald and Mercury to assist her against Amber?