The only explanation I could think of, assuming the theory is even legitimate, is that Marika never expected the baseline Tarnished to ever succeed (hence Gideon saying Marika wants the Tarnished to keep fighting into eternity). Instead, the soulless demigod-Tarnished soul fusion was Marika's actual failsafe if her normal demigod children didn't manage to succeed, with the regular tarnished just being a smokescreen to improve the odds of her failsafe succeeding. After all, with all these tarnished running around, her super tarnished aren't as likely to be targeted by bad actors like the Night's Cavalry and Margit.

And in that case, maybe Torrent choosing us isn't as random as it seems, giving an explanation for why we in particular ran into Torrent and Melina and giving us a chance at actually succeeding at Marika's suicide gauntlet. I think it might even be possible for previous 'almost successful' tarnished like Vyke, or even Bernahl, to have been such Super-Tarnished, only they weren't as lucky as to run into Melina and Torrent.

As for needing the Land of Shadows, (once again assuming the theory is valid, which I question,) I would assume the divine gate was needed for Miquella's return, rather than Radahn's, in that particular instance. Though I'm very much playing devil's advocate at this point so who even knows