Is the supplicant's saying in the Istikhara supplication, "If You know," considered a lack of etiquette with Allah, and does it imply a doubt in Allah's knowledge and power?

This supplication does not contain any impoliteness towards Allah Almighty. The Prophet, peace and blessings be upon him, explicitly stated therein that Allah is the Knower of the Unseen. The intention behind it is to ask Allah to ordain for the servant that which He knows to be good, to facilitate it, and to bless it, and to divert from him that which He knows to be evil.