a definite "softening" of the mother by the end of the story and her prioritising relationships over money. Greed, the key theme of the story, is what isolates her and prevents her from truly loving her children, and this is what drives Paul to ever greater excesses on his rocking horse. A more sentimental approach would make the mother realise what Paul is doing to himself sooner, enable her to live within her income and involve a "re-bonding" with her children, and especially Paul. Prizing relationships over money would definitely change the tale.

I'm going to assume you mean the literary sense of sentimental as in the 18th century writing where female protagonists are "guardians of spirituality and virtue." In this story, the mother, rather than being the spiritual center of the family, is well aware that "at the centre of her heart was a hard little place." The boy is the virtuous one who is willing to sacrifice in order to try and stop the whisperings that hurt both his mother and the entire family.

If this story were sentimental, I would expect the mother to either be the moral center of the family or to become the moral center. At the end of the story, her heart is not changed. The ending would also have to be more optimistic to fit into the sentimental genre. Right now, we can assume a pessimistic ending simply based on what we have seen. When the mother got the original monetary gift, she got a "cold, determined look" on her face and wasted even more money on things such as flowers and "iridescent cushions" while the whispering got even more desperate. Now that she has inherited sixteen times more money, the destructive nature of the mother's wastefulness is likely to just accelerate. So, the child is dead, there is little to no hope for moral improvement, and the woman is the center of this little disaster, despite the fact that she blames her husband for their lack of luck. All this would have to change before this could be called sentimental.

By "commercial writer," I assume you mean someone who is writing for television or film--something that would make money because it appeals to a wide-reaching audience. A commercial writer's primary interest is appealing to the masses so as to make a boatload of money on the show or movie--plus any outside marketing deals with fast-food chains or other products, of course.

Given that definition, the ending of "The Rocking-Horse Winner" by D. H. Lawrence might be commercially acceptable, at least in part. It might be okay for Paul to die, even though he is a child, because he kind of dies a martyr. He tried and tried to satisfy his mother's need for money, something he heard the house shouting at him for most of his life, but he could not do it. What other choice is there for him? Having done all, he has nothing more for which to live and nothing more to give the mother he loves.

That being said, the audience's level of anger and frustration with Paul's insatiable and selfish mother might be too much for them to bear, especially if nothing really happens to her. No one would be happy if she gets to live out her life with all the money she gets from her dead son, even if they know she is still miserable. Today's audience wants to see her suffer and feel as if she gets what she deserves. Yet, if the boy does not die, nothing is going to change and the audience will wonder why he does not die and break free of his untenable circumstances. Either way, it seems like Paul has to die to satisfy the audience.

Perhaps the solution, then, is to continue the story. Paul dies and his story ends; however, it continues once Paul's mother gets the grand winnings paid for with her son's life. Nothing good can come of that money, right? It was gained by the selflessness of her son, and her selfishness in spending it must somehow destroy her. 

If Paul's mother is somehow destroyed by her own greed and the money her son gave his life for, the audience will feel as if the boy's death has somehow been avenged and his sacrifice had some kind of meaning. This type of poetic justice is something most audiences find satisfying, and it could easily be done by simply continuing the story once Paul dies. 

The opening lines of the story are a good way to both begin and end the story:

[She] was a woman who was beautiful, who started with all the advantages, yet she had no luck. She married for love, and the love turned to dust. She had bonny children, yet she felt they had been thrust upon her, and she could not love them.... Only she herself knew that at the centre of her heart was a hard little place that could not feel love, no, not for anybody. 

The story already ends this way by implication; however, a commercial writer would have to make the woman's hard-heartedness more tangible for the audience. Any number of things might happen to this "unlucky" woman, but as long as she gets what she deserves, the audience will be content.