I saw that, too. Shockwave was Onyx . . . Shockwave was Amalgamous . . . how often did he jump back in time?
Maybe he is actually all of the Primes but each of them a different Shockwave that went back in time from a different universe and they all pretended to be a Prime to avoid being discovered? Thou apart of my headcannon for Shockwave is that he invents a time machine and also is able to clone himself, so when Unicron attacked Cybertron in the G1 movie, it was just one of his clones that died, the original carries on living, outside of time, watching everything unfold as the clones have recording devices that enable him to see everything that they had expirenced until that point when they die! If you wonder why he doing this, is simple, he is a seeker of knowledge and he has been infilencing various universes as apart of his experiments, he completely unchecked and unbound by the normal rules of other scieniests, he is almost immortal within this bubble that enables him to mess around with the Universes at will, the only reason Alpha & Vector don't go after him, is because he is apart of a Primus's will, he is seeding many universes with life or knowledge, so in his lust for knowledge, he pretty much becomes a tool for Primus without knowing he is a tool as he has lost all normal reason and common sense!