[The Might]

Just a small point, as far as the Orocarni are as along as the Blue Mountains were, as you said 900 miles

The Red Mountains were created in the very beginning before the first counting of time when the Valar laboured long and hard to order the world for the coming of the Children. The first design of Arda was to shape it in symmetry, so the Blue Mountains were as long as the Red, and if I remember the correct names we had the Iron Mountains in the north equal to the Yellow in the south. Please correct me if I got something wrong here.