I think the "The Seal contained the primals, and they were trapped by the Allag" theory is plausible, but to dismiss the entire question of why would Ifrit not then be an Elder Primal is overreaching (imho). I'm not discounting the possibility that Ifrit was sealed by the Allag, but I find more evidence than not that he wasn't around in the Third Astral Era. Every contemporary primal has lacked the elder description, and every primal of the Allag age has been called such. The absence of the black swan required for the theory to function is more than a "non-issue" (again, imho).
Odin and Behemoth are easily overlooked only if Odin was already the Dark Divinity (explaining why he was sealed) and the Meracydian brood is older than the Third Astral Era - nothing gamebreakingly contradictory, there, I suppose. But Belias is harder to make work with the theory. The summoner storyline tells us that Belias was summoned and defeated (at least once, probably multiple times) in the Third Astral Era by an Allag summoner. The memories of the summoner's victory, which enabled him to form and control Belias-egi, were encoded in his soulstone. This implies that, unlike Odin and Bahamut, Belias didn't have to be sealed away, and he wasn't that powerful. By extension, this likely makes Belias to his age what Ifrit is to ours - a contemporary primal. But, in our age, the Ascian refers to Belias' essence as that of "a slumbering elder primal." If the theory is true, it implies that Belias was ages older than Ifrit, but Ifrit was sealed whereas Belias was defeated by a summoner and his memory eventually lost to the ages.
It seems much more likely to me that Belias was a newer primal then and an older primal now, which means Ifrit would be more likely to be a newer primal now. I can't really find a way to make the IF BELIAS X, THEN IFRIT Y situation work in a way that's backed up by the story so far. It's like putting a puzzle piece in the middle when all you've completed are the borders. It fits, but will it still fit when we have more pieces there? Is it possible that Belias was older than Ifrit at the time? Sure. Is it possible that Belias was sealed later and forgotten? Also, sure. Again, I think the theory is still on the table, but we can't just write off the Belias v. Ifrit problem without adequate explanation beyond a guess, can we?
I don't know if it was blocked for a short time (or perhaps you later took a wrong turn on the way? It was a confusing location to reach, to be sure), but the crystal cave was open all the way to the day the servers closed and was a flag-trigger for two Seventh Umbral Era quests. The only thing that changed about it was that the three-sided shape in the middle of the largest crystal was removed and the symbol of Thaliak was carved into it. It was first seen in Like Father Like Son, where Cid referred to it as The Crypt of Xandes I and said that it was at Silvertear Falls. This implies that the crystal formation was actually the spire of Syrcus Tower, albeit in an entirely different location than it is in 2.0. The cave was later seen in Living on a Prayer. It's the location of Louisoix, Urianger, and the player's final interaction before the Battle of Cartenau, and Louisoix states that from that location he can "feel the gods stirring."
Now that I think about it, if the crystal cave was the 1.0 location of Syrcus Tower's spire, then it suddenly makes sense why one language referred to Crystal Tower's location as west and the other as east - perhaps one referred to the current crystal caves, and the other referred to the eventual location in 2.0...
Here's a list of the things in that video:
- 00:00 The Crypt of Xandes I (1.0 Only - See Above)
- 00:34 Dark Aetherial Node (1.0 Only; Dunno what it was. Weak gate? Void entrance?)
- 01:08 Gwyr-Aen (Still Exists)
- 01:31 Ishgardian Vigil (Still Exists; Design and size have been retconned)
- 01:59 The Falcon's Nest (Inaccessible in Coerthas Western Highlands)
- 03:03 Amberscale Rock (Still Exists; guarded by powerful elementals in 1.0 storyline; associated with dragon legend; looks like a dragon)
- 03:16 The Guardian Tree (Still Exists; heart of the Twelveswood)
- 04:00 White Tree (Unknown; Likely just an early Hedgetree)
- 04:13 Burnt Tree (1.0 Only; Burned by Khrimm in the storyline)
- 04:25 Lifemend Stump (Still Exists; Associated with Moogle Lore)
- 04:40 Gelmorran Ruins (Destroyed; remnants visible in ARR. Rocks missing, look similar to Amberscale, shaped kinda like tonberries)
- 05:08 Thal's Respite (Still Exists)
- 05:48 The Red Labyrinth (Still Exists in name, doesn't glow like that)