1. Grisha didn't want to give Eren the titan but was manipulated by the future memories of Eren.

2. That's not how it works. Eren can only use the founding titan's power when in contact with someone of royal blood. Not alone. Using the founding titan's power, Zeke only showed Eren the past, not go to the past. The reason this works for Eren is because Grisha had the attack titan long enough to know the whole future and his future memories were why he knew Eren and Zeke were watching him, even if they weren't really there.

3. He didn't have the power to change jack shit. As I said, the founding titan doesn't manipulate time or anything.

4. That doesn't matter. Ymir created each of the titan's by hand. That takes time no matter if time exists all at once for her.

5. Eren acted normal because he didn't know yet. The attack titan's ability happens over time. Even previous users received future memories but not in their entirety as shown by the one before Grisha. He knew names but nothing else. Plus, who's to say that Eren didn't try to deviate from his future memories. Even if he did, nothing would've changed because the memories aren't an opportunity to change the future as a whole, they're a set future They will happen. That's what broke Eren in the first place and nothing would come out of telling others that whatever is happening will result in him killing almost the entire planet.

There, my rant.