How can one reconcile the Hadith "He will be struck with a rock" concerning those who are sluggish in prayer, with the Hadith of Samurah ibn Jundub regarding the torment of the grave for those who sleep through prayer and abandon the Qur'an? Do these two Hadiths complement each other, or does each stand independently?

There is no contradiction between the punishment for one who rejects the Quran and the punishment for one who sleeps through the prescribed prayer. Both are punished with the same torment, as mentioned in the hadith of Samurah, which speaks of a man whose head is crushed with a rock because he takes the Quran but rejects it and sleeps through the prescribed prayer. Disobedient individuals may be punished with a single torment, or with different types of torment.