I like the way you're thinking outside the box.
But...I don't know. It feels like Oda just did this to give us a excuse to visit the Heaven/Astral Realm. That seems like the best place to evacuate the books to. But, I do enjoy a good twist. Let's explore some options.
Kid stealing the books doesn't really make a lot of sense to me. Kid's never struck me as a big reader. I don't see the motivation, myself.
I feel like any of the Revolutionaries are out of the running. Kurasu could MAYBE get the books out with his Soot Fruit. But, I think the Revolutionaries likely have their hands too full to have anything to do with this.
Same goes for any of the Marines or SWORD. They just don't strike me as being sneaky enough to pull this off under everyone's noses.
It could be other God's Knights or servants of Imu. But, "Mariejois being on fire" at the time, makes it sound like it was a all hands on deck situation back at Mariejois for most of the Elbaph arc.
But...I will say Ju Peter has been acting suspiciously, lately. Sweating, and looking nervous. And his "relative" Sommers, is on the island. I could see maybe Ju Peter "saving" the books, for his own reasons. Ju Peter is the most suspicious of the Gorosei. And Ju Peter may have somewhat rebellious feelings about Imu. So, maybe him. But, would Imu notice that Ju Peter was on Elbaph alongside everyone else?
Garling? Well...maybe. There's more to Garling than meets the eye, it seems. But, he also has a lot of the same problems as Ju Peter. He might have rebellious thoughts about serving Imu. But, being on the same island while Imu is possessing someone feels risky.
Shanks? Well, that'd definitely be a twist. And he was JUST on the island. And he has knowledge of how the Abyss works. He MIGHT have been able to set something up to teleport the books away, if they were in danger. And he MIGHT have a interest in the Ohara books, for himself. Shanks has been known to be more underhanded and tricky than some people give him credit for. It's not likely, but not impossible, either.
Now...Blackbeard, on the other hand, now that's someone I could see having a keen interest in Ohara's books. And he's DEFINITELY the kind of guy who would take advantage of a dire situation for his own gain. Sneakily and quietly taking what he needs and not leaving anyone the wiser. Plus, Biblo has a unique Devil Fruit, and Blackbeard is ALL ABOUT Devil Fruits. This definitely fits Blackbeard's MO. And with Van Augur's Warp Fruit, he could get the job done fairly quickly. Blackbeard could have been pulling a similar scheme as we saw on Egghead, but we never actually got to see it happen here.
So, chances are, if ANYONE actually stole the books and kidnapped Biblo, it's Blackbeard. I doubt that's actually what's going on. Biblo's probably just hanging out in the Astral/Heaven Realm, waiting for everyone to find him. But, if there is a twist with this, it's got Blackbeard's fingerprints all over it.
