Regarding N1 and N2, what does the "N" stand for?
As far as I can tell, the N comes from the physics behind turbines in general, whether they be water pumps or wind turbines, a subset of mathematics called fluid dynamics.  Many thanks to ROIMaison's answer for steering me in the right direction.

In Turbomachinery: Basic Theory and Applications, Second Edition, Earl Logan, Jr. provides a symbol reference list at the end of each chapter starting in chapter 2, defining $N$ as rotor speed, in units of rad/s; an angular change per unit time, and $N_s$ as the specific speed, a dimension-less quantity derived from a ratio of the flow and head coefficients.



N1 and N2 seem to be derived from both.  Wikipedia mentions the speed as a percentage paradigm as a human factors consideration, though as of 5/14/2015 there are no references to support this.  However, there may be a scientific basis behind this as well, since a change in N is proportional to the resultant change in airflow (i.e. thrust produced):