Re: the Alliance Raid, I have some thoughts:

In Final Fantasy XI, the Walk of Echoes is a specific area within the extra-dimensional Threshold, where everything the avatar Atomos eats is sent. Atomos is tasked with eating failed futures of Vana'diel. Hence why it's a Walk of "Echoes," since the real Vana'diel still exists somewhere. Sareel Ja even directly refers to the place as "the Walk of Echoes" at one point.

So apparently, when Sareel Ja slipped into the Skydeep Cenote gate after Zoraal Ja, he fell into a space between the shards, saw into the Walk of Echoes, and saw stuff that indicated the existence of a different version of Mamook and Gulool Ja Ja on Vana'diel.

Aside: electrope existing in the space between the shards is an easy thing to buy, since there was a ton of it there at the bottom of the Skydeep Cenote, and as other folks mentioned, the Strayborough Deadwalk in Living Memory was a section of the place lost during an interdimensional fusion experiment, which might have deposited it in this space between the shards.

As Cleretic has been saying, the idea that the universes of Final Fantasy are in a continuum that's separated by these extra-dimensional spaces has been used in Dissidia, Stranger of Paradise, and in several of the FFs themselves to explain when stuff crosses from one game's universe into the other; not all of the FFs share continuity or physics, but they DO all seemingly border on this extra-dimensional space. The Rift Between The Shards (which WoL passes through on the way to the 1st/13th) is one version of this, while the Interdimensional Rift (seen in Omega's simulation, as well as FF5/FF16) is a distinct space. I would posit that the Threshold from 11 is another iteration of the same space.

Which isn't to say that all of these spaces are THE same space. The High Seraph Ultima is supposed to come from an extra-dimensional space, which could mean some iteration of Ivalice, but it's vague enough that it could ACTUALLY be true or be unreliable narration. The incursion of the Seed of Destruction in YorHa: Dark Apocalypse is also characterized as an extra-dimensional push specifically into "a world on the brink of death" like the 1st. I don't want to assert that the extra-dimensional shenanigans necessarily means that NieR is as connected to FF14 as FF13, because that gets REAL silly REAL fast.

All that being said: if Omega is able to tap into the extradimensional space between FFs, that might explain why it was able to create simulacra of stuff from other FF games (namely 5, 6, and 1) rather than those concepts existing somewhere on Etheirys, sight unseen. So the idea that Sareel Ja, with the power of electrope, can create an emulation of Vana'diel for his own purposes isn't too farfetched given what 14's already done in the past.