Is the husband's father considered a mahram to the wife, like her own father, and is it permissible for him to marry her if she is divorced from his son?

There is no difference between a woman's husband's father and her own father in terms of being a mahram. Therefore, she is permitted to display her adornment in front of him, based on the Almighty's saying: "And let them not display their adornment except to their husbands or their fathers or their husbands' fathers." The son's wife is permanently forbidden to his father, a prohibition that is not affected by the son's divorce or death, based on the Almighty's saying: "and the wives of your sons who are from your own loins."