(1) Given Stalin's reclusiveness and ultra-tight security, how does the "Polish nationalist" get to him?
(2) On the more general matter of the succession to Stalin if he were to die in 1945, Andrei Zhdanov
http://en.wikipedia.org/wiki/Andrei_Zhdanov was often mentioned as a possible successor to Stalin at the time. The problem, however, is that his health was already poor, due in part to heavy drinking. My guess is that even if he does come to power (he could indeed become General Secretary, though if he does so it might be as part of a deal limiting the General Secretary's power) he will not last too long. (Stalin is sometimes accused of killing him, but it is just as plausible that without Stalin around, Zhdanov will die even earlier, since apparently Stalin did at least try to restrain Zhdanov's drinking [1]). After his death, a Malenkov-Beria alliance will (as in OTL with the "Leningrad Affair") probably take vengeance against his supporters. Molotov might serve as Prime Minister--a position not as powerful as it sounds (the fact that Rykov was kept on in the post well after the defeat of the "Rightists" and Bulganin for some time after the defeat of the "Anti-Party Group" is evidence of this). Khrushchev might survive the factional strife; he was helped by the fact that people tended to underestimate him, to think of him as a dumb muzhik, etc.
(3) I agree that in view of the party leadership's suspicion of "Bonapartism," Zhukov has virtually no chance. Certainly none of becoming leader by "legal" means, and to attempt a coup would be way too risky.
[1] At least according to Khrushchev: "Before his death, Zhdanov had been in poor health for some time. I don't know what he was suffering from, but one of his ailments was that he had lost his will power and was not able to control himself when it came to drinking. It was pitiful to watch. I even remember that in the last days of Zhdanov's life, Stalin used to shout at him to stop drinking. This was an astounding thing because Stalin usually encouraged people to get drunk. But he compelled Zhdanov to drink fruit water and suffer while the rest of us were drinking wine or something stronger."
http://spartacus-educational.com/RUSzhdanov.htm