MC 1.12.1
Forge 2485
In Control 3.6.1
(will switch to 1.12.2 shortly)

I tried to make hostile mobs neutral by setting angry to false, but it isn't working for me.
I wonder if it is not possible or if I am doing something wrong. Here is my potentialspawn.json:
https://pastebin.com/xQ2Dkx4f

That's not what angry means. 'angry' means that the mob is specifically angry for a player when it was just attacked. Hostile mobs are hostile even without angry

So is there a way to make hostile mobs neutral or passive then?

Not with In Control. That's also a lot more complicated because that involves modifying the AI of the mobs themselves. I can look into it but not at the moment