There's a lot of theorizing here, but we seem to be trending towards a particular view. This is my take on that view:
(1) Shademar is the cognitive realm. That is, it is the realm of thought. Everything that thinks would then be represented in the cognitive realm by something.
(2) Spren seem to have the ability to be conscious. That is, honorspren can become completely sentient, and presumably other spren have cognitive components as well. Thus they would have a representation in the cognitive realm.
(3) Shadesmar is connected to the real world, somehow. What affects one affects the other, and vice versa.
(4) Shallan talked to the bead, and then the goblet turned into blood. If we are to assume a simple causative connection between the two (probably best), we must assume that the bead in the cognitive realm represented the goblet in the physical realm. That would be why she appeared in Shadesmar holding it; she was touching it in the real world and so when she went into the cognitive real, she was still holding it. All that changed was the way she perceived it.
(5) Asking which was real, the goblet or the bead, probably doesn't make much sense. They are both representations of the goblet.
(6) It may have been a spren which she talked to, but we don't know. It fits, but it's hard to say. I find it interesting that a goblet would be represented in the cognitive realm, though.
(7) Stormlight apparently helps you stay afloat in Shadesmar. It probably gives you control. I wonder if it limits how long you can stay there?
I also suspect that none of these constraints apply to Hoid...