Is it possible for Peter III to King of Sweden and Tsar of Russia?
I was going to ask a variation of this question but as long as you started this. What if Peter's father (Charles Frederick of Holstein-Gottorp, Charles XII's only nephew) had gotten the throne of Sweden before his death (doesn't matter how) and become Charles XIII, making OTL Peter III, Charles XIV of Sweden - what happens in Russia?
Obviously Empress Elizabeth can't import her nephew and raise him as the Romanov heir and the only other Romanovs living (that we know of) were the jailed child Tsar Ivan VI and his sibling who Elizabeth had dethroned. If Peter III becomes Charles XIV does Elizabeth still seize the throne? And then what?
Also - he probably doesn't marry Sophie of Anhalt-Zerbst and there is no Catherine the Great in Russia.