Does Rex genuinely feel sorry for his behavior? Does Rose Mary ?

I don't think there is much guilt on the parent's part. Jeannette comes to the conclusion that she had to accept her parents as they were. Wishing they could somehow be responsible was merely fantasy. They were not sorry for their decisions anymore than they could have changed as people. 

Ironically, money seems to mean little to Rex and Rose Mary Walls as a means to buy necessities or to make a practical, settled life possible. Indeed, Rex's earnings at poker illustrate that he could be living in an apartment with running water were he focused on using the money towards housing. Furthermore, Rose Mary's tract of land, which Jeannette suspects might be worth one million dollars, could have dramatically altered the trajectory of life for her and her family, yet she never even considered selling it.