I tend not to think so. A thought proceeds from a thinker. It seems to me they cannot be the same thing. But then, on the other hand, the very idea of "proceeding" involves what you call linear time. But God is timeless. So in the end, all I can really say is: I don't know. God is not subject to "our" rules....
RE: "I would affirm the belief that a thought from God of something He wants to exist in spacetime is as guaranteed to come to pass as if it already exists" — the Holy Scriptures tell us that God uses His spoken WORD to effect His Will. So it seems to me it is the spoken WORD that carries the creative divine thought into efficacy, not the divine thought per se.
But then again, the Gospel of John tells us that in the Beginning was the Word, and the Word was God, and the Word was with God. The key to solving our puzzle is to be found here.
You wrote, "... can we say that each species did not exist in some dimensional reality before becoming limited to our realm of spacetime?" On my reading of Genesis 1 and Genesis 2, I would say that Genesis 1 refers to such a non-temporal (that is to say, "not yet in time") reality, wherein God created all things in heaven and on earth. But His earthly creatures are not instantiated as "physical" beings in the (dimensionally limited, from our point of view) "realm of our spacetime" until Genesis 2....
You wrote: "What if that which exists in our spacetime is actually less full in our spacetime than it would be in a more complex spacetime, or even a less complex spacetime?" This seems eminently possible to me.
The problem is, however, we do not know how many dimensions there are, particularly how many temporal dimensions.
I gather string theory takes a stab at ascertaining the number of dimensions there are. Last time I checked, string theorists were propounding something like 12 dimensions. But almost all of these "extra" dimensions are spatial, not temporal. And the extra spatial dimensions are seemingly rather bizarre, for they are thought to be "curled up" and of a size less than Planck length. So even if they can be "mathematically detected," it seems to me they would be utterly unobservable in principle; and so how to verify them experimentally is beyond me.
Personally, I think understanding the dimensionality of time may be more fruitful in solving the enigmas that bedevil science right now than positing more spatial dimensions.... But that's just a hunch.
And that's why, dear MHGinTN, I am so intrigued by your meditations regarding temporality: You posit time as having (at least) three dimensions — linear, planar, and volumetric. I believe you may be onto something here!
But again, that's just another hunch....
Just some thoughts, FWIW.
Thank you ever so much for your immensely thought-provocative essay/post!