
Damn, now that you say it like that, oof, ah well they did say they couldn't control the spirit stone? Right? Or is there something else, tbh I don't remember how it's used, but seriously...theyre really awful if that's how it is, I wonder if its just a mistake by the author who's too tired hh or is it meant to be this way???

The Freede lad can't "control", but not "use" it. That is, he can use the stone, but can't control the amplified darkness energy, which he doesn't have much control over even in a normal state. But that doesn't mean that the stone can't be used, he just touched the light stone and got "healed" automatically. And that seem to be his and his childhood friend's intention all along when they were searching for it in secret from the empress. In the previous life. So I think it would be safe to assume that it would be the same with the darkness stone and the light power users, no need for the Freede's successors to be able to use it on them actively.
As for the author... I think the author just didn't think it through. Like some other things. Like how the mc didn't have any realizations after she learned that her so called husband wasn't forced into marriage with her against his will and his preferences, but marriage to her was his scheme all along meant to use her position, completely disregarding her ruined life and her feelings. She didn't stop that tune of hers about how she's all evil and how he's her poor victim. You could blame it onto her blind infatuation, but Ethan didn't correct her interpretation either, though he's supposed to be smart.
Seriously, the Freede guy started liking her after the time reversal and confessed to her about his circumstances, because he new it would be unfair to hide it. Through it it should've been seen by the mc just how much he disregarded her before (and they got married long before she killed her brother, his doctor, didn't they?), but the mc didn't see anything, she just started feeling sorry about his situation all over again.
And this whole concept of him marrying her to get treatment from her brother... he was getting treatment from him all along without marrying anyone, so why do it? It can be deduced that he did it for guarantee that the brother will continue doing it in the future, but, first of all, that is too much to ruin a princess's life over something like this, and besides, if the aim was that the brother wouldn't be able to ignore his sister's husband's plight because he cares for her, then he shouldn't have married her all the more, if he only intended to torment her. Because if the brother cares, then why would he help his sister's tormentor? And if he didn't care for his sister, then why would he care for her husband? Again, he wasn't just cold and indifferent to her, he didn't even bother to fulfill marital duties at all... it was mentioned when she started dating Ethan how she didn't have any experience.
Truthfully, when it was first said that he married her because of his condition, I thought that it had something to do with her blood, even though she didn't have powers. Like, being close to a Millinare would stabilize him. Or intimacy would. Or anything. But it turned out it had actually nothing to do with her, only with her brother....
So year, I think that the setting is just messy, the author stuffed this story with all sorts of drama without properly thinking it through :) or at least that's what it looks like to me...
So... all this time, while the imperial family members were diying young for generations bc there were no Freede decendants that could stabilize their power, they had a perfectly functional dark spirit stone tacked safely in their treasury or wherever... without badging to help out their sovereigns they are supposed to be most loyal to... but then when a dark spirit user awakened among them he tricked a princess into a fake marriage just to secure her brother's lifelong assistance while slowly driving her insane with his attitude, then he and his accomplice tracked down the light spirit stone, intending to use it without informing the legal heir of this thingie, his wife...
What a nasty parasite...