All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Our answer to your questions will be summarized in the following points:
The First Point:
The sacrifice that is performed while intending its reward for the dead is like any other sacrifice; you may eat from it, donate from it and keep from it for yourselves, and it desirable to divide it into three parts.
We have already issued Fatwa 83800 clarifying that it is permissible to offer a sacrifice on behalf of the dead, but it is better to donate its price in charity (instead of slaughtering), unless it is known that the dead liked the sacrifice to be performed on his behalf.
The Second Point: Intending the reward of acts of obedience to the Prophet is not reported from the Salaf (the righteous predecessors), therefore, it is an obligation in this respect to restrict oneself to the religious texts, like the order of exalting his mention and asking for him Al-Waseelah (which is the highest position in Paradise), because acts of worship should be in conformity with the sayings of Allaah and His Prophet .
The Third Point: This kind of oath which is conditioned on achieving something is disliked, but an oath that is done just to get closer to Allaah, is desirable, like a person saying: "I will perform such and such action for Allaah" without conditioning it on anything.
For more benefit, please refer to Fataawa 86608 and 82716.
Allaah Knows best.