1) Yes, you should specify in your will that the 1/3 of the estate should be spent on obligatory hajj, lapsed khums, etc.
2) No it is not permissible to do that. In general, a person has disposition over 1/3 of their estate and there are specific rules to adhere to. The same rule applies irrespective of how much money a person leaves behind. She still only has a right on 1/3 of the estate. However, she can ask her inheritors to fulfill those lapsed obligations for her