There is nothing in Varieties of Faith, the Third Pocket Guide or the Code of Malacath that suggests that Orkey is not Malacath (all it says is that Mauloch is Malacath which is true). I just don't really understand why so many people think that Orkey is Arkay, there is no evidence to suggest that, only the similarity in their names. So why so many people believe this? It's not that I don't want to believe you because I am stubborn it just doesn't make sense to me what you are saying.
Challenging the status quo? I can respect that.
Anyway, I'll try and lay down all the reasons why I think Orkey = Arkay. And most of this comes from Varieties of Faith, since its our best source on the subject.
1. The name Arkay is similar to Orkey. A two-letter difference. This might seem trivial, but as far as drawing comparisons go, its important as this is a common hint in Varieties when drawing comparisons across pantheons. For example: Shor (Nord), Sheor (Breton), and Shezzar (Cyrodiil). And Lorkhan (Elven) and Lorkhaj (Khajiit). Also Stendarr (Cyrodiil) and S'rendarr (Khajiit), or Zenithar (Cyrodiil), Z'en (Bosmer), and Zeht (Redguard). Now obviously this can't be trusted all the time. It could be referring to the Orcs, which it also has a similar name to.
2. When VoF lists the Nordic gods, it lists them as so:
SKYRIM: Alduin, Dibella, Orkey, Tsun, Mara, Stuhn, Kyne, Jhunal, Shor, Ysmir, Herma-Mora, Maloch
If Mauloch (Maloch) is Malacath, and Orkey is Malacath, why would the same god be listed twice? Similarly, in Five Songs, both Orkey and Malooc (though not by name, see the Kyne's Son section) are mentioned. Why not just say that Wulfharth shouted Orkey to hell, if the Orcish chieftan was indeed the Orkey mentioned a stanza later?
3.Of orcs, Varieties writes:
Orkey (Old Knocker): A loan-god of the Nords, who seem to have taken up his worship during Aldmeri rule of Atmora. Nords believe they once lived as long as Elves until Orkey appeared; through heathen trickery, he fooled them into a bargain that 'bound them to the count of winters'. At one time, legends say, Nords only had a lifespan of six years due to Orkey's foul magic. Shor showed up, though, and, through unknown means, removed the curse, throwing most of it onto the nearby Orcs.
From this we can gather that Orkey was not originally a Nordic god, and given that his worship was taken up during Aldmeri rule of Atmora. So given this information we can postulate that the Nords recieved him from the Aldmeri pantheon.
4. Lack of further evidence.
The True Nature of the Orcs, Five Songs of King Wulfharth, and even Varieties (though to a much lesser extent) all draw some sort of connection between the Orcs and Orkey, the connection namely being the old tale of Orkey cursing the Nords down to six years old. But its important to note that none of the say that Orkey is Malacath, when we have numerous texts saying that Maloch is Malacath. And like I said above, Mauloch and Orkey are treated as distinct deities in the texts that they appear together in. Furthermore, in the Third Pocket Guide's detailing of Orsinium, which by far has given us the most information about Orcish religious beliefs, Orkey is not mentioned alongside Malacath (Mauloch), Trinimac, or Torug. That is, if he's a separate member of the pantheon.