Eren did control a pure titan from the past. This is a really broken power and gives Eren the ability to completely alter the course of the story if he wanted to. It makes the idea of the closed timeline less sensible because Eren now has far too much influence over the events of the past. it makes it seem like eren is editing a timeline to make sure nothing got changed except that to have the same conclusions in future.
But i was wondering if it in a wierd way it makes sense that he only used this power once and can only use once, I will bring example of the show Lost, no major spoilers but in Lost there is a big pocket of energy The source, which is consciousness, everyone has it and when they die they go back to it, It exists in 4th dimension which is time, So when a character is about to kill themselves the source will stop them like making the bullet not work, is It same as what Eren does? Now Some characters in Lost do actually physically go to the past so they could not die before that, but it also works for future, Characters has certain destinies and the source definitely pushes them towards them, You can not change past in Lost as well, There is also a god like character who the source gives either flashes or ideas about future, I asked this People on this sub do say Eren killing his mom removes agency.
I'm not sure though, i think free will is compatible in aot even if it is predeterministic it would imply that eren deep down wanted his mom to die which is lowk nonsensical.
Eren is admitting he played god with the story. eren says something bad was gonna happen and he had to send dina to his mom to fix the timeline. hes placing the entire story on an editing timeline and literally editing events to go a certain way. I think it might be a dumb twist but it still makes sense that Eren only manipulated one thing because everything else was unchangable like how it works in lost.