When we prepare an RMS Coach treatment, we are 100% faithful to the piano/vocal score. You will have the piano accompaniments as well as every vocal line for the entire show, including all songs, interludes, overtures, play-offs, etc. This means that RMS Coach is not just useful for the leads to practice the main songs, but also for the 2nd tenor in that close harmony section of the Reprise, for drilling the choreography during the Bows, or for timing the complicated blocking and scenery handling between Scenes 8 & 9.
You can use RMS Coach up until the last performance date of your run. The total time depends on when you first unlock it. Provided you have a valid performance license with Rodgers and Hammerstein or MTI, you could probably use RMS Coach as long as you would ever need it: 4 weeks, two months, a year... it's up to you.
For all intents and purposes they are the same product. RMS Coach is our in-house name for the product, but when it is licensed for Rodgers & Hammerstein shows it is known as AccompanEase®, and for Music Theater International shows it is known as RehearScore®.