I have to admit, I'm more than a little reluctant to be shoving the Carthax sector around myself.
Much though I would like to see excuses against having a combined event silenced, Carthax has been sitting where it is (in my mind, about 4000 ly to the galactic NNW of the Sabbat worlds, making it one of the most westerly points of Imperium space) for two and a half years - which means that location specific background has been written.
Sure, there's a few anatopisms, like the Tau. But it's a game, for Throne's sake.
If we have to be a bit generous about the geography for the sake of doing some cool stuff, then we bend a few rules.
The Tau were brought here by some particularly rogue Rogue Traders; Carthax manages to survive on the fringes of space because it has some spectacular warp links to further in-segmentum; all Carthaxian ships follow a large red line that lets them cross the galaxy in a 15 second scene that looks cool and saves runtime on the final cut of the film - whatever the reasons, I don't really care.
I can dig up any number of cases of these anatopisms.
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- Eisenhorn knowing about the Tyranids - aside from the fact this was 200 years before they had been sighted (D'oh), Tyranids are in the south of the galaxy, the Helican/Scarus sectors are in the northwest. Yes, it's only information, but still very far travelled. But then, Abnett.
- The suits used by Spyrers on Necromunda are Tau tech. And that distance is already 90% of that to where I choose to place Carthax.
- Almost any Space Marine chapter any time it appears in the fluff doing something other than getting massacred on their homeworld. To take the 2nd Armageddon War (north part of Segmentum Solar) as an example, the Ultramarines, Salamanders and Blood Angels were the chapters that came to the rescue.
Macragge - Sagittarius Arm, south part of Ultima Segmentum (~30,000 ly)
Nocturne - Cygnus arm, south of the galactic core. Again, Ultima Segmentum (~15,000 ly)
Baal - Cygnus arm, northeast of the galactic core. Guess what - Ultima Segmentum (~10,000 ly)
Given the treks involved, these were the three chapters that intervened? (Funny, also, that they're all First legions). Even if it had been the Imperial Fists, who are at least in Segmentum Solar, that's still many, many sectors away.
(The Space Wolves had a bit more of an excuse in the 1st War, as despite being in another Segmentum, they're actually closer.)
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For anyone who accepts these things as part of their canon, I'm not exactly sure where the problem arises. Nominally, yes, we get told that these trips should take years, decades, maybe even centuries, but GW clearly has never stuck to this rule themselves.