Black Leg I think most NPC drops or MA they teach are specific for players more but @Larsi Could answer this
I could be mistaken but yeah if it's for one it's for everyone that wants it unless it doesnt say it in the skills and abilities thread (though it's somewhat outdated)
You dont require mastery, just the necessary materials to create them.
Though the Power Suits and Armor provide their own buff, but there's no mastery as it goes off your normal stat with the exception of one or two with the buffs
I would slightly increase it to 6 though can go down to 3 months on one condition being; the user must be active and on an adventure or have been on one.
Or have the following:
1 Canon Adventure
Reviewed then if they leave CP or Marines they can still have Rokushiki as a MA
Hmmm nice catch, then yeah Rokushiki gets nullified though the user can create their own custom MA in that case or create some Fishmen techniques (I believe it was only 2-3 only) where it replicates Rokushiki.