"Why does Shinji rescuing Rei in Rebuild 2.0 cause an Impact Event?"
We already know that there are significant differences in the metaphysical biology of the Angels and Evas in Rebuild, and the mechanics of the universe are different than those presented in Neon Genesis. That having been said, I'm not using anything from NGE as the basis for my explanation. There is a bit of supposition of things carrying over from NGE to Rebuild, but even that stuff is sketchy at best.
So, lets look at the facts:
- The Angels are considered to be "of Adam(s)"
- Unit 01 is "of Lilith"
- All other Eva Units are either "of Adam(s)" or are one of the "Adams" (possibly Units 5 and 13 and definitely 9 as is explicitly stated, plus Unit 6)
- It is accepted that Lilith + Adam(s) = DOOM
- The term 'contact' is used most when describing "core interactions" including those between humans and an Eva (Yui...noooo...)
Now lets look at the specifics of the Zeruel situation:
- Zeruel is an Angel and therefore "of Adam(s)"
- Unit 0 is also "of Adam(s)"
- Rei is "of Lilith" (I'm actually going to present evidence against this so pay attention )
Now, with this established we have what goes on in the movie. Zeruel consumes Unit 0 (Rei still inside, obvs...) and several things happen:
- Zeruel is able to shift its signature to match that of Unit 0
- Zeruel partially changes its physical form to mirror that of Unit 0's pilot, and
- doing so does NOT create an Impact Event
Given what we see here, this alone is evidence to show that Rei in Rebuild is not "of Lilith." If she was, then the merging of Zeruel, Unit 0 and Rei would have caused Third Impact based on the accepted notion that the merging of Adam + Lilith = Impact Event.
HOWEVER!!!, there is also the idea that Rei may have been "encapsulated" within Zeruel's core to prevent such an interaction. This can be inferred based on how we see Rei as a whole within Zeruel when Shinji reaches in to rescue her. This does come across as a bit flimsy considering that Zeruel partially took on the form of Rei's body and to do so without interacting with her seems rather nonsensical. No?
So lets move forward a couple minutes in the movie to Shinji in Unit 1. We know from what we see on screen that Unit 1 ("of Lilith") beating on an Angel ("of Adam(s)") physically doesn't cause an Impact Event, otherwise Unit 1's battle with Sachiel would have ended the entire movie series... Then of course we have the physical interactions of Unit 1 with Shamshel and Sahaquiel, also with no Impact occurring. Mind you, there was no metaphysical interaction between Unit 1 and the cores of any of the aforementioned Angels. Sachiel detonates its core (to little effect), Shamshel's core never actually touches Unit 1 (just Unit 1's progressive knife) and Sahaquiel's core is taken care of by Units 0 and 2 (both of which are "of Adam(s)").
During the battle with Zeruel, Unit 1 kicks, punches, smashes, stabs, body slams and generally just beats up on Zeruel with no metaphysical interaction occurring. Round One Complete! Unit 1 Wins!
In this process Unit 1 manages to use up all of its SP and the round is over... 0:00! Round 2 begins and it's Zeruel at 25% HP with the comeback... until Shinji decides that this thing ain't over til it's over and then we have THE AWAKENING!!! Round Three; START!
Unit 1 reconstructs its arm using its AT Field then Shinji realizes he has the master control code for the Eva and ←→↑↓↖↗↘↙ A B HADOUKEN!!!'s Zeruel across the Geofront. Then things get interesting...
With Unit 1 in an Awakened state, suddenly stuff starts to get metaphysical between Unit 1 and the Big Z. Most notably comes the interaction between Unit 1 and Z's core. At this point we have the commencement of an Impact Event and we all know where this leads.
So what was this huge wall of text and misplaced video game humor for? I'm really just trying to establish a canon basis for "WHY." That the specific reason why the Impact Event is triggered is solely because of Unit 1 and not because of Shinji, Rei, nor Big Z. If Shinji had been piloting literally any other Eva then either he wouldn't have been able to rescue Rei at all or something entirely different (like the Eva absorbing Z and some freaky mutation $hit going on) may have occurred. The metaphysical interaction between the cores of two beings where one is "of Lilith" and the other is "of Adam(s)" is the "why" for [Near] Third Impact.
The completely, utterly and totally foregone conclusion: Shinji's life sucks from start to finish because Yui is a ho and Gendo has the emotional capacity of a baked potato.
Shinji: Thanks mom and dad!
This post is intended to be taken only slightly seriously and slightly humorously. Do with this post what you will.