1) The Mentor skill, even when mastered to Level 6, is still only giving a 5% Crew XP bonus at the end of games.
2) The requirement to master a skill to Level 5 was supposed to drop to 500,000, according to the News Article posted:
http://wotblitz.com/en/news/pc-browser/news/update-2_2-lvl-6-crew/
It has not. It is 624,000, as seen in screenshot here: