You're thinking about it from the wrong direction. They're not going to explain the raid to people who haven't done it, precisely because they haven't done it. The events have not happened. Their lack of sidequest completion has left reality in a slightly non-canon state where the Scions departed the First having only helped with the very preliminary exploration of the Empty, and Ryne hasn't even met Gaia yet.
This is simply how the game handles non-completion of sidequests – compare the quest that touches on your progress in Coil when Alisaie joins the party at level 60. You can see there are points where both Alphinaud and Alisaie's dialogue is loaded up with multiple script options depending on where you got to with those quests: didn't even meet with Alisaie; did the first stage; got far enough for Alphinaud to be involved; completed the whole thing.
So there is likely to be variable dialogue in the same vein to account for multiple stages of Eden, but then also they seem to have worked out ways to have characters conditionally appear in cutscenes now, so it's possible that Gaia will be there if she exists to be there. Or at least Ryne will talk about her, and perhaps she can be found elsewhere in the Crystarium.
Regardless of exactly how they acknowledge it, there will have to be a resolution for the MSQ that doesn't totally rely on Eden being the answer. Perhaps it gets handled through Ryne's power as the Oracle somehow.
My longstanding suspicion is that they're angling for further sidequest development on this front, and MSQ is just helping to get things in place for that. We could have a mega sidequest merger of role quests + Triad + Eden + post-MSQ thing for Zero.
So, what could we actually do with a massive voidsent prison that works by requiring us to perform dangerous and forbidden summoning rites to deliberately drag a powerful voidsent into the Source, give it a physical form to use and then attempt to imprison it?
Doubly so if you're thinking about using it on Zeromus, where the whole point is getting Azdaja out of it, not sealing the whole thing away.
One thing quite clear from the Void Ark storyline is that the ship was an unsuccessful prison. The voidsent got out and nearly wreaked havoc. We might be able to subdue it, but it would still be a matter of dropping a ticking time bomb in the Source.