10 year passion project
only came up with 15 episodes of material

Nishi interview translator here.
Many people seem to infer from my post that Nishigori pulled the ending out of his ass in the very last second, so as a counterpoint I want to mention that in a new interview, Hayashi said two things:
1. he wrote the script for the final episode together with Nishigori.
2. his work on the series was finished after the first few episodes of the series had aired.
Assuming that he didn't lie about last-second rewrites or anything, this implies the ending was decided on at least around four months in advance of the airing date of the final episode. Which of course doesn't change the fact that Nishigori had only planned out the first half of the series, just that scheduling issues weren't necessarily the main problem with the second half of the series.
Also, quick update on TL progress: Editing is about 40% or so done, but I'm considering to delay the TL for another day or so in order to translate a few interesting excerpts from two Hayashi interviews which tie in nicely with the Nishigori one.

The following is a lot of interpretation on my part with little actual evidence to back it up, so please take it with a grain of salt.
Regarding Hayashi, one of the most interesting things in the interviews wasn't was he talked about but rather what he didn't talk about, namely the romance aspect of the show. On what were six or seven pages of interview material, he mentions "love" exactly once, only to state that while it was important, it wasn't really his primary focus for this series. I don't want to infer too much from it, but let's that say that if he was actually the guy who wrote the majority of the romance aspects in the series, he sure managed to make it seem like he didn't really give a fuck about it.
This is something I've noticed with the other interviews as well. It's kind of strange really. There is a lot of talk about the setting and what inspired the series and what not, I almost get the impression that the writers were going for something entirely different and it happened to turn into one of the greatest love stories ever told by pure accident. Nishigori at least mentioned that one of his original ideas for the series was his guilty pleasure of a student getting seduced by a delinquent girl, so maybe it was actually him who pushed that aspect the most? Especially since he said he planned out episodes 1-15 specificially and he talks really, really fondly about episode 13.
I guess what I want to say is just, if I had read these interviews without having watched the series, I would have expected the tone of the series to be quite different from what we actually got. Whatever that means.
Once again, pure speculation on my part, please don't quote me on this. I'll have this stuff translated in a few days so you guys can make up your own minds.

What we do know for sure is that Nishigori is the one who was clearly in charge of episodes 1-6, 13 & 15, so exactly those episodes that made Hir02 fags fall in love with the show in the first place. So I was most likely wrong in using the word "accidentally" in particular. Rather, it might be more appropriate to say that I think the writers didn't anticipate that a large part of the audience would perceive the series as a love story first and foremost
Just wanted to clear that up.

Hehe, are you prepared for the most shocking reveal from the Hayashi interview?
Flower symbolism was actually Nishigori's idea as well and he named the Franxx.

Hayashi
The writing process was appearantly a pretty cooperative affair, with 6-7 writers sitting down together and sharing ideas and Nishigori having the last word. It seems Hayashi's role was more or less that of the actual scriptwriter, i.e. the guy who puts down everything as a cohesive script, but the contents of the story were already decided beforehand. He does indeed talk a lot about the setting, so I wouldn't be surprised if he was involved in that.
There was also this neat little question where he gets asked if there was an occasion where Nishigori shut down one of his suggestions, and Hayashi replies that in the early stage of the writing process, he was writing a tragic scene and got told by

Actually, Hayashi mentions that Nishigori exerted VERY tight control over episodes 1-6 and had pretty much everything planned out for them. And Yamazaki (mainly worked on the setting, also wrote the script for ep7) adds that with episode 7-11, many things were still undecided.

I believe a big part of this dissonance stems from the side cast, because in every single interview, the writers mention how they wanted to write a story with multiple protagonists and that they feel that they managed to achieve this very well. They actually believed that all of P13 had similarly important roles.

One of the major inspirations for this show and the 群像劇 they wanted to create (and spectacularly failed at) were the from Nojima Shinji, which Nishigori watched in his youth. With at least one of the dramas, Miseinen, the influence is instantly obvious if you open the japanese wikipedia article: the protagonist is nicknamed , written in Katakana just like Hiro's name in Franxx, another character's first name is

Miseinen
The universal theme of friendship, in all its vulnerability, tenderness, and purity, is explored in this drama. The flow of seasons forms a thematic continuity to the action: the severity of youth in summer, the sentimentality of autumn, and the frustration of winter. Together, this group of boys cope with common adversity, face the death of a friend, and prepare to deal with the world of adulthood.