But I think ICor 15:29 speaks of baptism for the dead and not of the dead although it makes the differentiation between they who baptized for the dead and not us so it could be used as an example of someone doing the practice to teach the principle of Resurrection as the main theme of the chapter.
All the commentary I've seen on the passage points out that it was a gnostic practice, which St. Paul points out, makes no sense as the gnostics had no use for the body-my body, your body, Christ's Body, His Mother's body, etc. In other words, their practice belied their preaching.
Not sure but I heard the Gnostics did practice such baptism for the dead. So the practice seemed to be there somehow in the timeline but maybe not by Christians or maybe some Christians adopted as an erroneous practice I guess all these are speculative and may well have happened.
So did ever the Orthodox church practiced baptism of or for the dead?
Not recently but a few centuries ago?
Is there any record sustaining such practices?
Just that heretics practiced it, but they aren't in communion with each other: Mormonism has an opposite view of the Body than the gnostics had.
and if not why would the Mormons say that the Coptics have indeed done so?
Copts aren't so well known in the West, except that they are known among the earliest Christians. So an Apostolic peg upon which to hang Joe Smith's heretical mantle, hoping that such claims won't be examined but taken on face value.
The reference to the practice, from the snippet view available, sounds like an act of desparation rather than regular rite.
What sources do they have or rely on which sustains the claim?
The sources they present are as follows:
That baptism for the dead was indeed practiced in some orthodox Christian circles is indicated by the decisions of two late fourth-century councils. The fourth canon (fifth in some lists) of the Synod of Hippo, held in 393, declares, "The Eucharist shall not be given to dead bodies, . . . nor baptism conferred upon them." The ruling was confirmed four years later in the sixth canon of the Third Council of Carthage.5
Some churches not represented at these minor councils did not feel bound to discontinue the practice. Consequently, the Copts of Egypt continued baptisms for the dead.6 The vast majority of Christianity, however, rejected proxy baptism. In some cases—as in the Roman Catholic faith—proxy baptism was replaced by prayers and masses for the dead. As early as the second century, prayers of this nature were known.7 Cyril of Jerusalem wrote, "Many say, what is a soul profited, which departs from this world either with sins, or without sins, if it be commemorated in the prayer? . . . We, when we offer to Him our supplications for those who have fallen asleep, though they be sinners, wear no crown, but offer up Christ sacrificed for our sins, propitiating our merciful God for them as well as for ourselves."8
Among the ancient documents that mention baptism for the dead, a large preponderance were written in Coptic, the latest form of the Egyptian language.39 Though no longer spoken, Coptic remains the liturgical language of the Coptic Church of Egypt. Though there is abundant textual evidence for this practice among early Christians in Egypt, some of my Coptic friends assure me that it is still practiced in the case of family members who die unbaptized. To date, I have found only one modern story of an Egyptian girl who was baptized by proxy after her death.40
It is likely that the Egyptians more readily accepted baptism for the dead because of earlier pagan practices prevalent in that country. Hugh Nibley noted that the Coptic pseudepigrapha is not only related to other early Christian literature but is also highly dependent on earlier Egyptian texts. Concerning baptism for the dead, for example, he gave many references to water purification in ancient Egypt, both for the living and the dead. Indeed, washing in water was essential to the resurrection from the dead in ancient Egypt, just as is baptism in the pseudepigraphic literature.41
Just because many heretics wrote in Coptic, doesn't make it Coptic Orthodox.