You really are married to your pro-Putin narrative, aren't you?
What the Nuland-Pyatt phone call is is a selective piece of personal communication cherry-picked for us by Russian intelligence, very likely with the help of their Ukrainian pro-Yanukovych affiliates. Phone calls between top diplomats and state officials do not "leak", they are intercepted by intelligence operatives.
What the call tells us is that in a situation where the pro-Russian president of Ukraine (and his party, the Party of the Regions) was apparently losing their grasp of the state apparatus in the face of heavy opposition and protests, the US officials in question are trying to fathom who in the (predominately pro-Western) opposition will have the best chance to rise to national leadership if Yanukovych and his party will fall due to the ongoing protests. By leaning on then-available information, and through meetings with opposition leaders, Pyatt and Nuland then come to the conclusion that Yatsenyuk, the leader of the biggest opposition party, and
a man the president himself had offered the post of prime minister in January 2014, would be the best horse to bet on in the race (so to speak). And then they direct their concrete efforts towards him.
This is the realistic reading of the discussion - the Western governments trying to position themselves to support the most potential pro-Western political faction leader should the highly unpopular pro-Russian government fall (which at that moment, we should remember, looked likelier every day).
To understand the discussion as "the Americans choosing the next Ukrainian PM" is just conspiracist pro-Putin thinking with no real evidence behind it. Remember, this intercepted call is
the very best the Russian intelligence and its helpers got to convince us that the ousting of the Yanukovych government was a US/Western plot - a cherry-picked, possibly edited piece of a larger discussion. As such, it is not very convincing at all.
As it is, even without
any Western influence, in the case of a crisis that potentially led to the Yanukovych government (where the Party of the Regions was the biggest party) breaking apart, the biggest opposition party, Fatherland, would have been the likeliest candidate to form the next government. And, again, Yatsenuyk was the leader of that second-biggest party in parliament.