1. You are right, the coccyx is the remnant of a vestigial tail, but still not entirely useless.
According to eMedicine, it is an important attachment for various muscles, tendons and ligaments—which makes it necessary for physicians and patients to pay special attention to these attachments when considering surgical removal of the coccyx. Additionally, it is also a part of the weight-bearing tripod structure which acts as a support for a sitting person. When a person sits leaning forward, the ischial tuberosities and inferior rami of the ischium take most of the weight, but as the sitting person leans backward, more weight is transferred to the coccyx.
2. It's not actually accurate for a child to be born with a tailbone appendage. The dorsal cutaneous appendage, or so-called human tail is a rare congenital anomaly with a prominent lesion from the lumbosacrococcygeal region. It is also often considered to be a cutaneous marker of underlying occult dysraphism. You are right again, there has been a report of a 9 - month old healthy male infant brought for consultation for a “tail like” structure in the lumbosacral area since birth.
3. No, it isn't. Spina bifida is a condition where the spine does not develop properly, leaving a gap in the spine. The spine consists of the spinal column, which is a solid structure made up of bones (called vertebral bodies) separated by discs of fibrous tissue. Behind this is an enclosed space called the spinal canal, which contains the spinal cord. The spinal cord connects all the nerves in the body to the brain. The canal is surrounded by arches of bone attached to the backs of the vertebral bodies.
In cases of spina bifida, something goes wrong and the arches of bone do not fully close. Sometimes there is only a gap in the bony arch, but at other times the spinal cord is also involved and does not form properly either. The skin over the arch can also either be intact or have a gap as well.
For more info on spina bifida, visit this site.
