Is it permissible for the husband to have intercourse with his wife after the end of menstruation by only washing the private parts without performing a full ghusl, and is she sinful for not being certain about the meaning of ghusl?

What you read previously is correct. It is not permissible to have intercourse with a woman after her menstruation ceases until she purifies herself. Therefore, you must repent and seek much forgiveness, and your husband must do the same.