Findegil: neat! I had a play with Ambarkanta V and the finished LotR map at one point, but didn't think of carrying through to this one and the Silmarillion map. (I wound up with the Orocarni wandering anywhere from the Misty Mountains to the Sea of Rhun, so my results approximately agree with yours.)
Comparing Ambarkanta IV and V to the later maps, it's pretty clear that Tolkien straight-up added the Misty Mountains to his worldview at some point. It would make sense for this to be when he decided The Hobbit was part of the Legendarium; he would then have added in the references to the Silm, and gotten the whole map pretty much how we know it.
The Misty Mountains are already closer to the Blue mountains than to these mystery eastern mountains. It doesn't seem implausible for the main continent of Middle-earth to actually end pretty close to the eastern edge of the map - with just another Beleriand-sized region to go. Intriguingly, this would make the Sea of Nurn the last vestiges of the Inland Sea of Helkar - and put Cuivienen somewhere in southern Mordor. I... have no idea whether Tolkien would have done that?
hS
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