The biggest issue I have with five Merlins in five universes at five times theory is that it makes Demonreach five times as vulnerable.
If it was created in one timeline, then it has to be unmade in that timeline. But if it was made in five, then a successful attack on any of the others would destroy it. That's five times as much trouble.
Five Merlins coming to make it in one timeline would at least avoid that.
Huge problem there... how would 5 merlins enter any one timeline without causing splinter universes by the action itself? The end result would be just as troublesome as they cascade outward. instead of twisting time together they'd fray it's continuing linearity.
See, I see it opposite, making it five times at the same time actually makes it harder to trouble with, as you must attack it simultaneously from all five in order to destabilize it's resonance. Hence I think why the attack HAD to come across time. It had to break time barriers in order to effect something that had already done so... if that makes any sense, if not let me know lol.
*would they not have to destroy the possibility of Merlin making the trip at all in order to negate any one timeline were he does so? probability combined with conservation of history, perhaps they change slightly with attacks upon their timelines but a new version still comes to a head of it?