I honestly can't fathom how the author thinks "not changing the present timeline" works. The way Harry talks, it's like as long as there's a temporary appearance that things when the way they ought to have, it's fine. Like, if someone's supposed to have died and they are instead hidden away from the world for all of the war. Except that then, those people are let out and will go off and do stuff that they wouldn't have done if they had died, which CHANGES THE FUTURE!