So in the “Ben Tennyson” wiki page, there is a paragraph, in Contents History, where a character called Dr. Logic is mentioned
Ben had an encounter with someone named Dr. Logic when he was 19 years old. Dr. Logic disagreed heavily with the way Ben handled using his Omnitrix. Blinded by his goal to "fine-tune humanity", Dr. Logic eventually forced Ben into a scenario with an impossible morally-correct choice, challenging everything Ben stood for as a hero. All evidence of the event was wiped from Plumber Archives, and Ben has not talked about him since. The event changed Ben's ways of how he used the Omnitrix from that point forward.
For me, this is probably the event that made Ben so against using Alien X, like I’m really “The event changed Ben's ways of how he used the Omnitrix from that point forward.” implies that this event is either
A) is the reason Ben is so diverse with his Aliens. Or.
B) the reason Ben has a “Powerhouse Playlist” and is so against Alien X.
We have strong evidence that suggest Alien X will appear in chapter 12 but I feel it won’t be until after a rest match in which both parties have time to discuss plans so maybe someone, presumably Danny, gets Ben to talk about the encounter and then he convinces him that in certain situations, Alien X should be used, or something like that. Then let’s say Ben uses Alien X but Vlad shows us the true power of ecto-plasm. But then, Vlad, on the verge of victory uses his final blow against Divinity and saves the day.
Extra: after rereading this whole thing I realize how inaccurate this is but I am fairly certain we will get some more info on Dr. Logic, maybe not the whole story but definitely some more info.