What is meant by (if he sleeps through the prayer in the Hadith), and does this include one who hears the Adhan while exhausted, and is it permissible for him to miss congregational prayer and pray individually?

The hadith referred to is the long hadith of Samura regarding the dream, in which it states that the one whose head is crushed with a stone is he who takes the Qur'an but then rejects it and sleeps through the obligatory prayers. This warning is specific to those who intentionally abandon prayer. As for someone who is overcome by sleep without negligence, they are not held accountable, due to the Prophet's (peace be upon him) saying: "There is no negligence in sleep," and "The pen has been lifted from three... and from the sleeper until he awakens." However, for someone who hears the call [to prayer] and intentionally abandons prayer and sleeps through it, they are in great danger. Congregational prayer is an individual obligation, and whoever is extremely exhausted to the point where attending it becomes difficult, may be excused from it. However, patience and striving to perform it in congregation is more preferable.