Is it permissible for a wife to uncover her chest to her husband's father, or kiss him on the mouth, or let him touch her breasts, under the pretext that anything not considered awrah (private parts) is permissible to be seen or touched?

It is not permissible for your son to kiss his wife on the mouth, as this is only permissible for spouses. He should kiss her on the forehead or head, not on the mouth, as Imam Ahmad stipulated. It is forbidden for him to touch her breast or look at it, even if there is no fear of temptation, because the breast is awrah (private part). It is permissible for him to touch her extremities, such as the head, arms, and feet, if there is no fear of temptation, and it is not permissible to look at anything else.