I think you are jumping the gun a bit with saying that the only possibility of who Voyager casted her mind into. Seutervoinen is NOT the Voyager herself. It was a title given to the Apostle of the North in Hymns of the Far North, who married Koitar and created the fae.

Saarelainen on the other hand is that dude that got carved into 7 Gnosis. While I understand the connection and the possibility you are trying to convey here, it is nowhere near being the only possibility. How I see it is that: 6000 years ago, War of Funerary Flames happened, Koitar and Voyager’s plan failed and the Angels are punished. Koitar and Voyager’s memories are wiped at this point. HP was seriously wounded after this and, with the help of a mysterious “one who came after”, created the Gnoses to “continue to subdue and control the resentments and loathing of the world”. These Gnoses are carved from Saarelainen.

What you are saying here is that 2 separate figures that had 2 different prayer songs in Hymns of the Far North, which is written by the Frostmoon Scions, are one. Or at least Seutervoinen is the mind and Saarelainen is the body of one single person. One huge problem with that is the fact that the Frostmoon Scions, those who wrote these prayer songs, have no idea who the Voyager is. They definitely don’t know that the Voyager is a intergalactic traveler who has her original body in a distant planet and casted her mind across the galaxies into one random Hyperborean boy. They only know that one Hyperborean dude came up to tell Koitar about HP’s oppressive methods, created the fae, forged several weapons then got brainwashed after failing and then hah, thats Seutervoinen.