(a) It dominates her thoughts because aside from having children and having a husband, it was one of the biggest things to ever happen to her in her life. When she is approaching death, she knows that it is going to be something left unresolved and offering no answers. Also the jilting is the only thing in which George ever harmed her, and the it is proven that people will remember the negative more than the positive.
(b) In the last two or so paragraphs Granny reaches out for the heavens for a sign from god. She wants to know what her future will be, and she doesn't receive an answer. The presence of the preacher and the fact that she had been jilted by George make her think that now God is jilting her. Both of these experiences leave her with grief and are negative, something that most people don't want.

" this darkness would curl around the light and swallow it up. God, give a sign!
For a second time there was no sign. Again no bridegroom and the priest in the house. She could not remember any other sorrow because this grief wiped them all away. Oh, no, there’s nothing more cruel than this – I’ll never forgive it. She stretched herself with a deep breath and blew out the light." (5)