I really like the idea that 12 and almost all the Ivalice games are in the same universe as 14.
Sure, the landmass doesn't make sense, sure the names are kinda flipped around a bit. but It's been more then a few generations and this is a land that has been utterly wrecked by environmental disasters because of the Ascians.
Everything from Vagrant story to FF12 is directly referenced and most damming of all in my mind, is the fact that there are 14 reflections of the source as opposed to 15.
Meaning that if we consider each of the worlds as based on (if not being) one of the numbered titles, either one of them does not exist or one of them merged with another.
I can basically tell you that Ivalice as explained in Tactics/12/Vagrant story/etc existing in FF14 makes no sense. We already have a history for FF14's main setting (Aka "The Source") and Ivalice as described just doesn't work in that setting. They're all references and Ramza, Mustadio, Orlandeau, and Agrais being Warriors of Light and blessed by Hydaelyn as explained in FF14's "Return to Ivalice" storyline doesn't work in their stories in Tactics.
Also King Delita is actually Ramza's dearest friend in FF14's version of events and he laments and despises Ramza being forgotten to history in a flashback, and it was stated that Ramza demanded to be forgotten to try and bury the truth about Auracite. This doesn't work in Tactics' story as Delita doesn't have the same dynamic that he does in FF14's take as Delita is more distant while Delita is far nicer. Also Fran is a different character in FF14's canon as she is a leader of a resistance movement, Balthier, Vaan, Penelo, and Basch don't really exist, and Gabranth is a Garlean pure blood with his own specific army completely divorced from his position as a judge in FF12's version. They're just far too different and all connections are just cute little references.
We've seen the 13th shard and we know some of its characters even if you do the side content and none of them align with Lightning's story or setting, and it was also an FF3 reference as Cloud of Darkness is there and the Crystal Tower is tied to it, so it has nothing to do with FF13. The first shard we go to in Shadowbringers also has nothing to do with FF1. Also late FFXIV Shadowbringers spoilers for those that care as the
FF1 Warrior of Light is Elidibus who is effectively a primal, and the WoL title is heavily implied to be a story that was started by him to ensure people know a hero is out there so they can control the narrative of good and evil for their own designs. Not the WoL as explained by say Dissidia's story for him.
Also their is no evidence that merging shards works how you think it does. If a shard merges with The Source, the shard just fades away forever and goes into fuse with The Source as its aether flows back into The Source. It changes The Source on a "magic fantasy chemical" level, to be simple because FFXIV lore is weird, but the worlds don't literally merge like you're implying there is no information that says this.
tl;dr: This idea doesn't work, it is too contradictory within FF14's established canon of how its parallel worlds work and what is in them. They're just references, don't over think it.