/jp/ - Otaku Culture

I have a problem with these contractions. I interpret them like this

開く
開かないのだと
開かないんだと
開かんだと

I did the same with となる　in the second pic, but when I checked the English translation, it is not a negative form ("we will become Agito"). The change from last u-syllable into a-syllable lead me to think it was ならない

What am I missing here? What's the difference that marks the second contraction as positive?