What should I do if I receive a shroud that has slogans and writings corrupting to the creed, as well as invocations like the Jawshan supplication? And how can I atone for allowing it to be placed with the body under the earth?

Washing Muslim deceased is a great باب (door/means) to good deeds. Whoever washes a deceased and conceals their faults, will be forgiven forty times. As for the reprehensible acts you see on the shroud, such as seeking help from other than Allah, you must change it with your hand if you can. If you cannot, then with your tongue. If you cannot (change it with your tongue), then with your heart. If you see the name of Allah or the Qur'an written on the shroud, it is not permissible. If you can remove it, that is good. Otherwise, advise with your tongue. And if not, then denounce it with your heart.