We didn't use the crystal from the hydaelyn trial after story. The crystal used itself. which sadly has the implication that Hydaelyn to some form was still concious within the crystal to act upon itself and the 'world' of the 13th. Which is odd that in the story something like that would take place since using the last of Hahadealin and the remnant aether would effectively kill off Hydaelyn as a whole, but we know that characters don't die unless the fans like them.
Its either a very big subverted expectations plot twist or just more nonsense to the story that won't be explained. I think the writers have a hard time bringing the WoL into action for the plot of the story in FFXIV without a guiding force like Hydaelyn where one, hydaelyn is the reason for the story to move in a certain direction and it is more accessable to get the WoL to do things because Hydaelyn said so; and two, its a way to control the power of the WoL in a manner that limits the amount of "power creep" that the WoL as a character can display, which is the explanation for the use of Dynamis against Metion and the keeping of the WoLs power without an active Hydaelyn to empower them.
The story of FF4 and the 6.2 patches are just a blending of themes. It allows the devs to use prior FF games for fan service, as well as breing a way to introduce the plot of 13th and move that story forward. Its easily understandable that some people wouldn't like that because its very limited and narrow in the scope of potential world and universe building, but it does help prevent things like incoherent stories that aren''t fully fleshed out like they have been in WoW for some time now. Its not perfect in FFXIV but it works to some degree and makes the 'story' experience more cohesive for the player when they just sit back and experience the story woithout to much thought.
Endwalker gets lots of praise because of players doing that when if you thought/think about the story, its very flawed.
As for the FF4 theme itself, I like it when things like that are brought into the game. The music is as it almost always is top tier remakes, and its nice fighting theme park bosses like that. I loved the Ivalice raids solely because of FF Tactics. And would love a Mystic Quest theme park raid tier as well.