Question on Swedish Monarchy

Okay so in 1743 the Swedish parliament agreed to make Adolf Frederick of Holstein-Gottorp heir to the Swedish crown in the Treaty of Abo. But if the Russians push for an annexation of Finland like the Betuzehvs wanted instead, then who succeed Frederick I of Sweden

Might the Swedish estates decide to offer the throne to Landgrave Wilhelm of Hesse-Cassel, the king's brother and heir, provided he accepted a largely powerless role?
His son and heir, Frederick, is already married to Pss Mary of Great Britain, the second-youngest daughter of George II. He has not yet converted to Catholicism.
 
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