How would "Winter Dreams" be different if Dexter had married Irene Scheerer after all? Would he have been able to achieve his life's ambitions with...

The questions that relate to Dexter's choices between Irene and Judy capture the possible tension between happiness and ambition.

Fitzgerald makes a clear case that Dexter could have found happiness with Irene.  Marrying her would have allowed him to experience contentment: "He knew that Irene would be no more than a curtain spread behind him, a hand moving among gleaming tea-cups, a voice calling to children."  Had Dexter married Irene, he would have had to settle down and raise a family.  As a result, she offered a stability needed for sustainable happiness. This development changes the story's arc.  Dexter's "winter dreams" would have been a passing phase in his life.  They would have had less of an effect on him because the daily realities of raising a family and domestic responsibilities would have crowded out his painful yearning for Judy.

Nothing in the text indicates that Irene would have threatened Dexter's professional ambitions.  She wanted nothing more than to complement Dexter, wanting only what was best for him. However, it is clear that Irene would not have been able to cure Dexter of his ambitious desire for Judy Jones.  While Irene would have wanted to be everything for Dexter, her limitations were unavoidable: "The thing was deep in him. He was too strong and alive for it to die lightly." Dexter's desire for Judy was so firmly embedded that no matter how hard Irene tried, she could not satiate it.

Fitzgerald is saying that there might have to be a choice between ambitions and a happy family life.  There are times when both of these run counter to one another and are incompatible.  Sacrificing one for the other is the only way to resolve this tension. Dexter chooses his ambition for Judy over the stability Irene offers.  As a result, Fitzgerald illustrates the danger in selecting personal desire over domestic responsibility.