One albeit vague scenario for Kollontai path to power I could see would be the following:
First and most importantly Lenin gets assassinated before he enacts the ban of factions. Than Stalin is taken out of the picture, either he dies or becomes disgraced for his failure of protecting Lenin
Kollontai meanwhile remains an important force in the Worker's Opposition and the Zhenotdel, the women's section of the Communist Part.
A major scandal compromises the mainstream establishment of the Worker's Opposition or whatever the the faction may be called in the thirties after some sort of stable internal party democracy has evolved. Drafting Kollontai as their candidate is seen as the only option for the Worker's Opposition to gain/remain in power. An outsider and yet simultaneously an elder statesman she wins the internal party elections an becomes the Soviet Union's leader.
As for Krupskay, the only way seeing her coming close to power would be utilizing her rivalry with Stalin. An interesting piece on their relationship can be found
here.
The highlight of the article is probably this exchange:
After Lenin’s death Stalin started ignoring Krupskaya completely. It remains unclear how Stalin made Krupskaya move into the shadows. According to the British historian Robert Conquest, Stalin said to Krupskaya that if she didn’t stop criticizing him, the Party would announce that it was not her, but an old Bolshevik, Elena Stasova, who was Lenin’s real wife.
“You know that the Party can do that,” Stalin allegedly said.