Is the intention behind washing the face from ear to ear in Wudu to wash from earlobe to earlobe, or to wash the entire ears along with the face? And does the ruling regarding not removing dirt under the nails that prevents water from reaching (the skin) in Wudu apply to Ghusl?

The lateral boundary of the face extends from the root of one ear to the root of the other. Thus, the ears are not part of the face; rather, the face ends at the beginning of the ear's root. The white area between the ears and the sideburns are included in the face. The established Sunnah (definition) is that the ears are wiped along with the head. The earlobe is part of the ear's root, so it is wiped along with it and not washed with the face.

Slight dirt under the nail is excused in both wudu' (ablution) and ghusl (definition) (ritual bath), as both are purifications of the same type. However, removing dirt makes the purification and cleanliness more complete.