Is it permissible to offer Faatihah for Prophets and Walis by names or in general for all of them through one supplication? Also, can one offer Nafl Salat for credit (Sawab) to Prophets and Walis or dead relatives?

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

One should not intend the reward of good deeds to the Prophet or other Prophets and Messengers, may Allaah exalt their mention, as it is not confirmed that our righteous predecessors did so. Had there been any good in it, they would have preceded us to it. Indeed all the good is in following their way and methodology. It should be noted that all the good deeds of the nation of the Prophet will be on the scale of his good deeds on the Day of Judgement. For more benefit, please refer to Fatwa 87520.

Nonetheless, there is no harm, Allaah willing, in intending the reward of good deeds to other than the Prophets and Messengers, may Allaah exalt their mention, according to the most preponderant opinion of the scholars. It might be that it would be more appropriate for a Muslim to stick to what is confirmed, i.e. supplicating for them and giving alms on their behalf. In this regard, please refer to Fatwa 84090.

It is permissible to restrict the reward to some Muslims or to all the Muslims.

Finally, it should be noted that there is no evidence on specifying chapter Al-Faatihah in gifting the reward, so it is better not to specifically choose this chapter. 

Allaah Knows best.