How is it possible that a Nissan Tech can’t find the cause of code PO300?

Hello, the first thing to learn here is what P0300 means. It means merely that there is a "random misfire" being detected by the engine's processor at some point, and that there is not any specific information to be learned from that code. Typically that code is accompanied by other codes which indeed assist a technician in troubleshooting your concern. My guess is that your technician is working with Nissan directly and their technical assistance to resolve the concern -- and if he isn't, he should be. Consult the manager of the service department you are working with and discuss your concerns first hand. It could be that there is some kind of updated calibration coming out that will resolve your concern; I'd probably try to lose some of the frustration that you are feeling and realize that the dealership service department is in the best position to help you.

Check engine light has not returned ... How do you know a trouble code still exists? .. SOMEONE using a code reader? Was the person that told you the “tech does not know what is causing the fault code” in the position to actually be giving advice?