Is it permissible to pray facing a slanted wall that deviates from the true Qibla by 30 degrees, in order to make it easier for worshippers, knowing that deviation from the Qibla in this direction may not invalidate the prayer according to some fatwas?

If the deviation from the Qibla is 30 degrees, then your prayers are valid, because the obligation is to face the direction, and this deviation does not go outside of facing the direction. A slight deviation does not harm, unlike a significant deviation, which is deviating from the direction, such as facing East or West for the people of Medina and those north or south of Mecca, or for those in the West, such as America, and deviating from the East (their Qibla). The evidence for this is: "What is between the East and the West is a Qibla." However, it is preferable to adjust the Qibla of the mosque or the rows so that the worshippers face the Qibla without deviation.