Reality is a grounded realm, naturally implementing many laws which prevent the fabric of energies to change pattern according to user perception. Things remain the same here for it is an unbalanced realm, deprived of many fluid advancements that other realms may have in exceptional quantity. One of which, is Dreamspace.
Why Reality is such a grounded realm is still open for debate. It may have something to do with The Shade Theory (which is the theory this realm suffers an instability, sort of like a 40% 60% thing in positive and negative energy flow, which would also explain how entropy operates here). It all rests in the energy type. We all know no matter what creature or entity you are in this Universe, energy has different forms and the patterns therein can slow or speed depending upon the form. Reality implements a unique type of spacetime, time is slightly bent, it seems (see The Arrow of Time theory), and in turn this affects the space here very strangely. The form of space is macroscopic, composed of elements from afar regions of realms (this means whilst the space still has an almost void core, many things have clung to the shell, and developed over time or quick collection). The two together create a slow pattern. The space is complex (but not necessarily advanced), and the time is curved (therefore creates a slight entropic effect on the space). If it were not for the time being as it is, Reality would be a largely static pattern, not changing unless it was changed by an outside or inside force (depending on one's location).
I will save you attempted explanations as to why time is distorted here, but it is not far fetched to hypothesize this realm has been damaged by something. And whilst I do not like to say damage, as that implies wrong, I must make it known that I personally think it is damaged, and wrong, because this slow entropy dictates the very nature of anything within that realm, stealing away your choice to change or remain the same -- which I believe to be a violation of freedom, and a negative one at that, for everything goes downward rather than upward, the entropic effect decays things more than it does ripen them. And I find that unbalanced...

This said, for only the very beginning part of your post, you can see I could provide much elaboration, but I shan't get ahead in one particular area, let me regard where you say: "It is possible that dream material isn't destroyed but rather stored in a medium similar to the way that physical matter is stored in the matrix of spacetime. Then where can my dreams be found?"
It is entirely possible, as I theorized, though I did not do so in great detail, particularly. I threw the disk concept out there, but only to show that realms are finite places, and do not cover the whole of the Universe (which would be impossible anyway considering infinity). A better example would be to line three fat disks up (kind of like pies), exactly the same size, and then bring them all together, so they resemble one disk -- merge them all, if you will. They cooperate together, existing with each other. But they cannot be accessed globally. Instead they are accessed individually, just like one cannot see the infrared spectrum until they switch on the appropriate technology, which could be viewed as a portal, if you grasp that concept.
This could also answer where your dreams can be found: on an invisible dimension of Reality, it can be technically perceived as separate to Reality, hence named Dreamspace, but it is really just a different form of spacetime that runs alongside Reality's form.
As I understand, Dreamspace possesses entirely void space, capable of being subjected to a void if the dreamer/s wish it so. And the time there -- that flows all directions, no entropy is present, things remain the same until changed. A special form, indeed, and the patterns it is capable of creating are equally incredible.

Now, about the apple thing... I found a mistake (in my mind) of perception fairly early, I shall suggest a different perception of energy for you, but do not regard it as objective fact, this is my perception.
Now, generally, humans regard energy as a finite source or base frequency to The Universe. They believe energy can be lost and created, when in my beliefs, energy merely fluctuates. The fabric of energy remains completely static, think of it as trillions of dots, all spaced out but close together, -- now throw a square through them, the square itself does not move, but rather the dots are manipulated temporarily to take the form of a square 'moving', like a large ripple, inflicted by an external source (the entity that threw said square). On a larger scale, you can see even rippling water does not move, outward, but rather upward : each particle of aqua playing its part in reaction to infliction. So when one creates an apple, it is better to think of one manipulating what is already there to take the image of an apple. No energy is drawn from somewhere else, and no input was required, it merely became something else. Once the apple is there, it never disappears, just as a flock of seagulls do not 'disappear' when they go behind a mountain. Many of your questions in this sector can be answered with relativity. But if you had a particular question you desired answering here, don't hesitate to request.
Needless to say in Dreamspace, memory seems irrelevant, if you find your way back to the apple, it shall be the same, as the form of space and time in Dreamspace do not allow entropy, and your memory only plays a minor role in dreaming.

"I think that there is a poorly understood connection between the memory of something and it's dreamspace location relative to the observer in physical space. Perhaps memories of conceptualized realities are highly complex algorithms for their locations and fundamental attributes."
I have to say, this surpasses my comprehension, whether it is my poor knowledge of the English language, or your poor selection of words, I just cannot grasp this proposition. I may revisit this when I am better rested... As for the latter part:
"If it were possible to share the exact memory of a dream place with another person, would they be able to consciously meet you there in dreamtime?"
I believe so. Despite memory playing minor roles, it can often be the key back to many locations in Dreamspace. Since Dreamspace is infinite, you are going to needs some keys, these keys are stored in memory partially, I believe. However, sharing exact memories is sadly impossible. Memories are like binary, you would need a method of transferring memories from one brain to another. Technology I doubt exists presently.

Lastly, my techniques are somewhat complex. I feel I would be posting something longer than this just for the basics as to how I dream and how I return to places. But if you actually read all of this and still desire to know, I shall attempt to. Hopefully quite concisely...
Thanks for sharing your thoughts, anyway.