This is my take on this after a bit of reading:
I think the problem is that the influence of Etro in FFXIII, at least how you are made aware of it from just the game, is so subtle that it's easy to miss completely. Maybe I just don't pay enough attention and am a bit dumb, but I didn't realise a lot of it until doing further reading.
If you watch the scene again, with more of a mind as to what certain things symbolise, then it falls into place. The crystal shards that float in the air are a bit tricky, since they have two origins (appears when people die and at Etro's interventions) though both relate to the Unseen Realm. The gate to the Unseen Realm is more obscure. When I first played the game, I thought that eye looking thing behind Orphan (it appears when Orphan says 'Lo! The Day of Wrath is come!') was part of Orphan itself, rather than the gate to Etro. Didn't realise what it was during the ending either. But the gate stays there until the end, when it goes out after Cocoon is saved (the light in the centre of Cocoon that goes out during ending). And finally the brands being scorched as a sign that Etro intervened.
The crystal shards are very difficult to connect with Etro on your own given what the game alone says, which talks primarily about their relation to human deaths. The gate is at least mentioned in-game, though only as the door souls pass through rather than it opening when Etro acts. If they had shown some other instances of it, like people dying and the gate appearing, it might have helped make it clearer. (Though I suppose the gate might not physically manifest itself when people die, only when Etro uses her powers. Which still means that it could have appeared more when the eidolons did.) The brand is probably the easiest one to figure out on your own, as it mentions in the Analects a few times that Etro reversed Fang's transformation and it's a matter of connecting the dots from there (Fang was Ragnarok > Etro changed her back > Fang's brand is scorched > Scorched brands are something to do with Etro interfering).
With these things in mind, it's easier to work out what's going on. When Fang becomes Ragnarok and attacks Orphan, you see the gate appear behind it. This tells you that Etro is about to do something. Then you see Lightning, Snow, Sazh, and Hope back in human form. Their brands have been scorched, just like Fang's after her Ragnarok transformation hundreds of years ago. Since Etro reversed Fang's transformation in the past, and now the l'Cie have brands similar to her's, you can deduce that Etro was responsible for the transformation. (Of course, this is a deduction based on knowing the conclusion and signs already and just piecing them together using what is shown in the game.)
When Lightning says 'maybe it was more fal'Cie smoke and mirrors' (ファルシ得意のまやかし as Sazh's line after seeing Dahj and Serah's crystals shatter, Lightning says ファルシ好みの幻かもな; both were translated as the same thing in English), it sounds like it might be talking about them turning into Cieth and maybe that wasn't real. It's actually about them returning to human form. It's kind of a joke on Lightning's part, saying maybe this is just fal'Cie trickery, showing Fang her friends safe and back to normal again to mess with her. (The transformation is definitely real, caused by the shock of what Fang was planning on doing and the critical hit she inflicted on the rest of them.)
That's not to say that there is absolutely no involvement from the l'Cie. Out of the 'miracles' involving Etro (not the false ones from Dysley), there are two types: ones purely down to Etro, and others that occur due to the l'Cie's strength of will in addition to Etro's power being involved. The events of Fang's original transformation fall in the former category, but the ones seen during the final chapter (the l'Cie turning to normal and Fang and Vanille's Ragnarok saving Cocoon rather than destroying it) are the latter. Etro may have helped, but it was the l'Cie who made it happen.
anyway here are some trivia things i found while looking at the ultimania:
- if you look at the signposts and billboards in Oerba, you can see a few Gran Pulse phone numbers. There's 09987 for 'fresh fish', and 120242 for
Strong Strange Technology ('Strong' was crossed out on the concept art itself). There's also a Pulsian Internet address on a billboard for 'Cycle Shop Marvoric'! (
www.csom.co.gran) Evidentially there was a form of Internet on Gran Pulse at some point.
- Lightning's Blaze Edge weapon (which is a kind of railgun), for some reason, has Pulsian writing on it. It says 'invoke my name I am spark' and 'blinding instant', but says 'Lightning' in Cocoon lettering near the hilt. Other weapon inscriptions include 'super voltage' and 'hand made' on Lightning's Omega Weapon, and 'snow' and 'bear' on Snow's Wild Boar (wondering if that was meant to be 'boar' on the coat as well).