@trochira To answer your question about the processing rule, yes, you are correct about answer A, because you are using an eventID. That eventID can only be used ONCE, and nothing else can be done with it ever again, which is how Adobe maintains the uniqueness of its value. We actually ran into this when we found out that orders were accidentally going through with duplicate IDs when we were first learning how to do eComm. Does this get to the heart of your issue?
As I understand it is not possible to serialize events via Processing rules. This is also mentioned in this SDK document . You can opt for VISTA rules, crafted by Adobe Engineering Services, but that is a paid option. (VISTA rule info... ).