Does the presence of very small avocado mask particles on the hair and scalp after washing affect one's ablution (wudu')? And must the prayers that were performed be repeated?

Wiping the head during ablution involves wiping only the visible hair as a concession, without needing to reach its roots. The presence of very small amounts of hair mask on the hair or scalp does not affect the validity of ablution, as long as it is not visible. A small amount that is visible is excused, similar to a small amount of dirt under the fingernail or what is found in cracks in the feet. Therefore, your purification and prayers are valid, and you are not required to repeat them.