Page 9 already, eh.
As above. Controlling space itself, pretty much. In that context, Doom was, as said, linking control of the Negative Zone with absolute control over space that, added to mastery over time, would give him full domain over spacetime. I say this because obviously dimensions and space synonyms, when it comes to physical places
That specific scan, as said prior, only serves to prove that the "juncture of many dimensions" shown in Fantastic Four Unlimited was indeed the Crossroads of Infinity. My focus here is mainly on the scan from Fantastic Four #51, where Reed mentions "the world of limitless dimensions."
So to answer your question: Yes, I would say that. The relevant scene itself starts with Reed entering a 4-dimensional reality (And narrating that and every other detail, in old comicbook fashion), and then as he finally reaches the edge of Subspace, he reaches "the world of limitless dimensions." The continuity between both statements, as I see it, would indicate that the latter statement refers to spatial dimensions indeed, and this is supported by how in a later venture to the same Crossroads, he describes it as a "juncture of many dimensions" in a context tied to spatial dimensions (And by how control of the Negative Zone is directly equated to control over dimensionality, later on)
As said prior, I don't really think of the idea that the Crossroads intersect all realities as mutually exclusive with the idea that those scans are referring to spatial dimensions. To reiterate: Dimensionality is obviously a feature of the universes, and since the Crossroads are the single realm where all these universes meet, it naturally borrows from the dimensionality of each of them.
Also I don't quite understand what all this fuss about universes within universes is all about. I don't think it's realistic to suggest that "Macroverse" in the context of that specific Silver Surfer comic really refers to the realm introduced a while later. For the matter I don't get the "flowery language" argument that I've seen, either. For reference,
here's a statement from the same run, where said universes are described as holograms folded inside of more holograms, with the character in question even referring to them as created by consciousness.