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What if Donella wasn't just cursed to never enter the library, but she was cursed to never be able to use/work with magic again? Whenever one of her soldiers retrieved an enchanted item for her, it always worked for them but never for Donella. That's part of why she needed Hugo to be the one to go on the trials. Of course it's because he's smart, but it's also because he's an artificer, and enchantments and whatnot are kinda his whole thing. She could send her entire army into the library on her behalf, but none of them would know where to go from there. Hugo would.
Also, I like to think that Hugo often helped Donella when combining said magic objects into her weapons. Donella and some of her workers would make the weapon, but Hugo would add the item and make sure it's in perfect condition for their clients. Mainly I just need some mother/child bonding. Like Donella eventually letting Hugo work almost alongside her, and Donella teaching young Hugo more advanced things about engineering (though Hugo would just use this knowledge to upgrade Olivia, but don't tell Don that!)