Mountains of Mordor

[Kuruharan]

The Ambarkanta map in which the sea of Helcar is located where mordor is located in the later maps is from 1930! At that time there were no Hobbits, no ring, no Mordor. In UT it is stated that the Druedain passed south of Mordor and then turning north in the First Age. Also, there are references in Home to the Sea of Rhun in the first age prving that the Ambarkanta map had become invalid after the LotR was written.

Perhaps, but it still provides something to go on.

then the rest of Middle-earth would have also been changed, and we don't have any sources stating that. If the plate action was that strong, then a couple of other rifts should have opened up in Eriador, and caused even MORE destruction there. We haven't heard anything about that.

This is true, and perhaps there was. There was not anyone of particular importance living there at the time, so we don't have their stories. There is a certain lack of records from this area in that time. (And the description of the Breaking is a bit vague.)