Forgive me for my ignorance on the subject matter, but I have a few questions for now. 

(i) If causal powers are denied, and God is the only cause, what purpose do objects serve for the Occasionalist?
(ii) How does an Occasionalist account for gradations of sensation or being?
(iii) Doesn't the very act of receiving the Occasional cause imply something of the sort of passive potency? If it doesn't, does this serve to demonstrate a new category of potency is needed? If it does, this brings me to another question. 

DanielCC wrote:

Let us assume the truth of direct realism. With this in mind how are we to handle the manifest properties of a physical substance e.g. its colour and visual shape when un-perceived unless we admit that it has the power(s) to cause these perceptions on our part? The Occasionalist can answer that it's not the substance causing the perceptions but God - very well we answer but that is implicitly admit the substance does nothing, in which case why posit it in the first place? Why not just have God as the sole cause of our experience of the phenomenal world? We can easily apply Kim's argument against property-dualism to the entire of the material world.

(As can be seen with Hume and the aftermath of his philosophy to empty a substance of its casual powers is to deprive it of its very nature and being)

This is assuming the Occasionalist will still grant that we have causal power; if this is denied then things get even worse. To be the possessor of casual powers is the mark of a concrete being. Yet on the Occasionalist's account only one being has true casual powers and thus only one concrete being exists in toto i.e. God. Ergo we are stuck with pantheism and all the assorted problems such as modal collapse.

​(iv) If this is true, that the total substance-hood of a thing is denied, then what exactly is the subject of reception of the Occasional cause, i.e. what sense does it make to say that 'i' am affected by it? 

Ibn Rushd (i.e., Averroes) would have agreed with you. In his Long Commentary on Metaphysics IX.3, he writes (emphasis mine):

Contemporaries of ours [i.e., Ash'arite theologians] claim that all existents have one Agent [who acts] without mediation, namely God. For them no thing may have its own specific [causal] act which God impressed upon it. But if existents do not have acts that specify them, they will not have essences proper to themselves either, because acts differ only through diversity of essences. And if essences are eliminated, then names and definitions are likewise eliminated, and that which exists comes to be one thing."

 

This I admit, does not make sense to me, most likely because I am unfamiliar with Occasionalism and am not properly equipped to deal with the literary format of the Islamic writers, however,

(iv) If the Occasionalist is wanting to opt for the replacement-as-change, given that there is only 'one' thing(whatever it is referring to), "what is being replaced with what" seems to me to be an unintelligible question, maybe I'm wrong, but I don't understand how this works, so I need help here.
(v) Does this 'one' thing refer solely to God, and God alone? For if that is the case, given that there is no such thing as real change, and God is the sole affector, who/what exactly are the receptors of the Occasional cause? But God cannot be replaced, I hope this is a misunderstanding of mine, so I'm happy to be reprimanded at any step of the way in order to learn more about this.

I know these are a lot of questions, and maybe they have tons of misunderstandings, so I have no problem in being reprimanded, feel free to tell me what I've got 'right,' if I've got anything right, and to tell me what I've got wrong, if I've gotten any or all of the questions resting on incomplete understanding of what Occasionalism is actually arguing for.