Shinji may be triggering all his, but the inactions of others are giving him a wide birth to do so.
Aside from what others have said, I think you're severely
overestimating what a kid (hell, a person period
) in Shinji's position would have been capable of absorbing in the brief time before Mark.09 attacked. People accuse Wille of telling too little, but consider the possibility that, instead, they told him too much to start with!
The inherent difficulties of relaying information to someone who's come out of a 14-year coma and represents a potential danger, while needing first and foremost to tend to a base of military operations that could and does come under siege by matter-vaporizing hordes at any time, aside
... I'd say the only real mistake Wille made was parading Shinji around on the bridge (which doesn't make sense any way you cut it; Shinji should have remained in proper containment until his identity was confirmed). Everything else is IMHO readily understandable if you're not limiting your perspective to a single person's. Shinji may be the main character but the world actually doesn't
revolve around him.
While the film's structure nefariously encourages myopic identification with Shinji's plight, this being Eva
and all we can anticipate that this is a quite deliberate setup for a long-delayed payoff. The sooner one can liberate themselves from this "poor mistreated lad who doesn't need to learn anything from his mistakes because he did no wrong" narrative, the more prepared one will be for what's coming.