-The twisting nether has time just as we do, it has a past, present and future, our present is the same as the demon's present.
-Because of this, demons can only invade us in the present, and they invade every single timeline at once
-if one being becomes a demon, then all of his counterparts in every single timeline, also becomes a demon, this event becomes fijated in time
-As we have never seen a demonic Velen, then we can safely assume that to this point in time, not a single Velen has fallen to corruption
-Every single alter ego of a being, become one in the nether after dieing (if he was corrupted at some point)
-Wod was in the past until Kairoz connected it to our present, so when Guldan was corrupted, he was corrupted because of the Kiljaeden of that timeline, but after Kairoz meddling he was now talking with present Kiljaeden, to the legion it was like if a new opportunity appeared out of nothing.
-The legion can´t travel through time but they are cross dimensional beings, once a demon enters reality, he enters all timelines at once, first, many beings became one in the nether and now the same process can occur backwards, one being in the nether can become many in the timelines
With all these rules there shouldn´t be any problem with the whole: one legion - infinite timelines