I would ... the Tevs are ruthless to a fault, but not completely stupid 
Remains to be proven by any empirical data. <wink>
(Talking about society as a whole. Steele is obviously a brilliant (if ruthless) person)
I honestly haven't played the original AoA campaign in awhile so I can't really comment, but I guess wouldn't be surprised if there were discrepancies as the story has ... evolved. 
Yeah, i don;t think this has anything to do with 'evolution' of the story. They basically say the same things, but differently. Also, the Vishnan version seems to be 'propagandated' - it sums a very long response/explanation from Shivans as "We won't" [let them pass].
On the other hand it may be the Shivan version is more truthful. Or perhaps they are BOTH true. Or neither?
Anyway, Whatever the aftermath of GTVA victory may be, the aftermath of GTVA welcoming their long lost brothers and sisters with beam fire will have (already has) much more dire repercussions. (Why hello there Mr. Sathanas 312, please form an orderly queue at the jump node, right behind Mr. Dante)
That definitely sounds more like propaganda than a mere difference in perspective. I really wouldn't trust either of them. Although it is odd ... what's broken down on the cosmic scale since the implication is that Shivans and Vishnans are no longer cooperating in that other universe and now ours (and how much comm is there between universes for each species)? Of course there are a huge amount of implications from that little exchange between the Vishnans and Shivans about who is what and why. From something Ken said, it made it sounds like the Vishnans could order the Shivans to do something - that they were going order the destruction of man by the Shivans, but from the exchange and other information, the Shivans are actually older than the Vishnans.
I am also surprised the GTA security council reached the decision they did when it was strongly theorized in the GTVA by both Vasudans and Humans that the Shivans might be attracted to war even without the above knowledge. It seems an awful risk to then go to war even just suspecting that - even if you think humans are better off with your form of government to face the Shivans and if you are afraid of alien influence within the other government. Whatever their end game plan for Sol actually is, they're risking extinction by doing this. Unless they actually do know what Shambayla (sic?) and decided it was worth the risk of extinction to stop it, but I dunno: they don't seem to know about it and it's hard to imagine what else could cause them to take such extreme measures...
Of course that's also the trouble about theorizing what the GTA's occupation of Sol would be like, as long or short as it may be before armageddon. I'm not sure we know all the motives yet.
Even the Shivans ... it's odd. They want GTVA defeated/destroyed but also seem to want Laporte to stop the Elders and Shambayla. It's hard to see what exactly their angle is - especially, as aforementioned, with respect to the Vishnans. Also the Fedayeen made mention that the Shivans were more complicated but their counterparts motives were easier to understand (or maybe I flipped that around?). However, they didn't seem to elaborate on that.