As for Question No.1
-Look closely and you will find that they are saying the same thing, its just a matter of wording (jep. is kinda vague in a sense of just using the word "differ" without specifying details just like the ICAO did)....
but what they are trying to say basically is when you need to deviate from your altitude below or at FL410 the deviation should be 500 ft as long as your new altitude will be below or at FL410 but as soon as you need to be higher than FL410 then it should be 1000 ft (i.e FL390--->FL385 or FL395 BUT FL410 would be either FL405 or FL420...
As for Question No.2
No, FL 420 is not used for routing because RVSM LIMITS are from FL 280 and 410 inclusive, so above FL410 the vertical separation reverts to the non-RVSM 2000 ft., consequently, FL420 is out of the question for routing in any flight direction.(your next available FL for normal routing would be FL430 & FL450), so FL420 is merely a contingency for the FL410 for the facts stated above (Above FL410, you descend 500 or climb 1000), so think of it as a boundary limit for MNPS routes just like FL285 is used as contingency for FL290 (which is the actual minimum FL used in MNPS airspace)....so basically FL285 and FL420 form the brackets or the absolute limits that hold the MNPS airspace that you cannot go below or above even in abnormal situations.
Last edited by heggo48; 18th September 2013 at 00:05.