If God, as Creator, has the right to destroy His creatures, He certainly has the right to order some of His creatures to destroy others. Therefore, in such a case God would not be acting immorally. It is not abnormal for God to work through proxies, as He does when He performs good acts through us.
The "voice of God" objection is a good one, but not insurmountable. This doesn't present a difficulty for Christians who, at a minimum, accept the close of Revelation with the canonization of the Bible. The Bible indicates that Christians must not murder (although killing is sometimes acceptable). The Bible also indicates that demons can appear as angels of light. Therefore, private revelation must be treated with skepticism and must conform to Biblical revelation. Therefore, a private revelation commanding murder would have to be rejected. Christians generally recognize that God used the nation of Israel to kill evildoers at least partially as a means of testing and forming them as a people, as when He ordered Abraham to offer up his son Isaac as a sacrifice.
The Catholic position is even more solid. The Church has determined that Revelation closed with the death of the last Apostle. Moreover, the Church, as the living Body of Christ, must be the final arbiter regarding the validity of private revelation. Private revelation must conform to both Scripture and Church teaching. Therefore, a Catholic would be obliged to disregard a revelation which commanded murder. The Church regards the circumstances regarding the slaughter of the Canaanites as special, as described above.
Hardly. History clearly demonstrates that the Catholic Churches' ethics have been as morally relative as God's.
The Bible also discusses a prophet who offered his virgin daughters to be raped and sexually abused as a method of appeasing the mob outside his door.
Sophistry and silliness.
I'm glad that you agree that the alleged "revelation" to commit genocide should have been rejected as the voice of psychosis, since authorization for it was obviously was not to be found in any holy writings at the time.