Time for a Sidequest
So first of all, I think you and the Eldritch Knight learned a valuable lesson: the Deck of Many Things is a trap.
To restore some balance to your situation, I would recommend a plot hook that focuses both upon the Moonblade as well as the Eldritch Knight whose soul has been lost.
By RAW, the EK is incapacitated, but I can tell you now that's a crappy story for your situation and your player. I recommend a DM ruling that is similar to applying the Death CurseDeath Curse in AL, whereby the EK has to be extremely careful about dying lest his death be very permanent.
I recommend you deploy a plot hook to recover the EK's soul driven by whatever BBEG you want. I don't recommend the arc take more than a level or two to complete. As part of completion of the plot hook, obligate that to release the soul, the player with the Moonblade needs to sacrifice some (or all) of the Moonblade's power to release the soul back to the EK.
Perhaps the soul is trapped in some sort of special box that requires immense magical energy to open that can only be fueled by an artifact like the Moonblade. Within the box, you can also include an appropriate item for the EK.
This is also an excellent opportunity to utilize lair effects to enhance your story. If your BBEG exists in a lair that represses excessive magical power, the Eladrin may find that their Moonblade's power (and other artifacts) is suppressed during this quest. Lair effects can be whatever you choose, so given your overall goal on this quest, this may not be unreasonable, and will help to mitigate the Eladrin blowing out the other players during the quest.
This solution does require that the players at the table be of a collaborative mindset. If you suspect they may not be, I would recommend a brief discussion regarding the issues you raised here prior to your next game and indicate that you intend to rebalance things in-game. This lets the players know they need to trust you, but doesn't let them know how or what is going to happen.
I think this might be your best middle ground since it'll be a story you write focusing upon 2 characters in the party, but everyone can assist on. If done right, it could be something your players talk about years after the fact.