Does the inventor need to understand and/or disclose the scientific principles of the invention that make it work?

No, the inventor does not need to understand or disclose the principles that make his or her invention work -- as long as it does work.  In fact, if the specification provides the information needed to make and use the invention, the patentee's theory as to why it works can be completely wrong. See ​Process Control Corp. v. Hydreclaim Corp., 190 F.3d, 1350, 1359 (Fed. Cir. 1999).