No. Chiari malformation is associated with many conditions, some of which have abnormal chromosomes that would show up by amniocentesis. The malformation itself can be seen by fetal ultrasound but not usually at 16-18 weeks when visualization of fetal anomalies might prompt recommendation for amniocentesis.
No. Amniocentesis will be positive if the nervous system is malformed and exposed to the spinal fluid. A chiari type 1 abnormality means that the cerebellar tonsils are sitting below the exit point of the skull called the craniocervical junction. The brain and spinal cord are otherwise normal and as a result the amiocentesis will be normal for a chiari type 1.