I can't speak about NRE, but I can offer some first hand perspective to some other points.
Promotions for officers are almost guaranteed through LT. 2 years as an Ensign -> LT Junior Grade. 2 years as a LTjg
-> LT. After that, there are "typical" gates, but someone who was an officer in their previous life would be better off answering that part. All of this provided you don't do anything stupid like get in some trouble with the law (DUI was the biggest culprit) or sleep with the help (i.e. Officer and Enlisted personal relationships are a no-no).
OIS is Officer Indoctrination School. That's where the Navy sends doctors, lawyers, nurses and other "professionals" they bring into the officer corps. It is NOTHING like boot camp or even OCS. The Navy realized that those types of folks don't appreciate being yelled at by Marine drill instructors too much
You learn the basics of the Navy structure, uniforms, regulations, terminology, rights & responsibilities, etc. As to whether you would go to OIS or OCS, GC (or resident expert in this area) would be able to answer that.
And lastly, follow GLW's advice. These forums are LOADED with valuable information waiting to be discovered. All you must do is put forth the effort to find it.
Best of luck to ya!
