Is it obligatory to wipe the hair itself on the head during ablution, or is it obligatory to lift the hair to wipe the scalp? And what is the ruling on previous prayers if the latter was obligatory?

Wiping the head during ablution (Wudu') is obligatory, and scholars have differed on the sufficient amount for it:

Hanafi school: It is obligatory to wipe a quarter of the head (the forelock). Maliki and Hanbali schools: It is obligatory to wipe the entire head. Shafi'i school: Any amount that can be called "wiping" is sufficient.

What is considered for wiping the head is wiping what is on the skull, whether it is hair or skin. It is not necessary to lift the hair to wipe underneath it; rather, wiping the hair is sufficient, even if it hangs down over an area where there is no hair, as long as it has not extended beyond the head's boundaries. Based on this, the previous prayers are valid.