I think the explanation is far easier that timelines and dimensions merging.When dealing with maps, cartography etc, there is one huge element I think a lot of people are overlooking. We all assume reality is linear and unchanging. From my personal experience I can say beyond any shadow of a doubt that the general outline of our continents have shifted from what it used to be. This might be my personal reality or a shared reality with others - but South America was most certainly a thousand miles to the west of where it was. Australia was further South, New Zealand was to the north east of Australia, Japan NEVER invaded China through Korea because Japan was way further north etc etc.
Someone on the Mandela Effect forums even specifically pointed out the change in time zone when calling his relative who lived in South America.
So if in the span of one lifetime a map can change so significantly- and the history along with it- imagine what happens in the span of 500 years. You'd end up having all kinds of different maps and even stories of different continents that wouldn't add up if our world was linear and unidimensional. Hyperborea, Atlantis, Lemuria- there's room for all of them in a non linear multidimensional model of earth.
We have all different kind of nasty people who control the education system and wrote our encyclopedias. what we know about history is only 5% of it all blurred with lies, so we basically have no history at all. Everything is one big movie script. This explains the mandela effect.