I know this stuff is more confusing than Russian Calculus.
Yes, week 20 you can go back to "full cycle." According to the study and our experience NO, Myostatin will not be a factor at this point and the cycle will baseline from there on UNLESS you raise doses then Mystatin gets back into the picture because of acceleration of the gene activity causing increased protein synthesis, which your body will signal myostatin to slow it down.
If you don't raise doses at this juncture of the cycle then your body gets used to the new level of activity and combats myostatin.
Don't ask me why the human reacts to steroids this way..
The bottom line, stay on full cycle full time or do like what has already been mentioned.