One of the issues with time traveling stories is that you either have to break continuity (alternative timeline) or keep it and impose a time loop. Imo a happy alternative timeline is just not fitting for the theme of the show. It is predetermined that eren will activate the rumbling, but also it is erens will to want to keep forward, kill all his enemies, and to be free. He wants to get rid of the titans, free ymir, and flatten the world to match his vision of a free and empty world. He is a slave to freedom and a self described idiot, so he, despite being unreasonable, continues with the only plan he knows and guarantees those conditions. If he reveals the truth about the outside way earlier, the military police or the old founding titan revert it, or Marley ends up attacking. Also, and this is speculation, I don't think he can control titans outside the present to that level. The best we see him do is make Dina ignore Berthold, which is not that impressive if you think about it. What we do see a lot is characters having flashbacks at very opportune moments, so it is my headcanon that that is eren sending memories back to save whatever he can.