There are
no multiple timelines involved in Elpis. Those events are located in time prior to the split between the "bad future" timeline and ours.
When we travel there, it's not "our Elpis" with a second copy left untouched for another WoL to visit. Everything that happens there forms the past of both timelines together, and our actions become part of the other timeline's history as well as ours.
I tried to map it out as a better explanation. This is still rough but I hope it makes sense.
The key thing to get your mind around is, from the perspective of our current timeline, our visit to Elpis occurred in our past – and from the perspective of someone in the bad timeline, our visit (ours specifically, not an alternate self's) is part of their timeline as well. There doesn't need to be a second alternate event.