Does major ritual bathing (ghusl from janabah) suffice for ablution (wudu), and to what extent is this statement and its derivation sound? Does this include Friday ghusl and ghusl for cleanliness? What is the ruling if a person touches his private parts during ghusl?

If a Muslim performs a complete ghusl for janabah, this ghusl suffices for wudu, because the minor ritual impurity is subsumed under the major ritual impurity, whether one performed wudu before it or not. The evidence for this is the Almighty’s saying: (And if you are in a state of janabah, then purify yourselves).

As for ghusl for cleanliness, cooling, or a sunnah ghusl like the Friday ghusl, it does not suffice for wudu, because it is not in the same category as ghusl from janabah. Wudu must be performed after it if wudu was not completed during the ghusl.

If the person performing ghusl incurs a minor ritual impurity during or after the ghusl, he must re-perform wudu for prayer.