Maximum rate of climb - think ballistics, how long will you wait after the engine fails for gravity to take over and enter autorotation? 6-7000 fpm can be achieved. i.e > 60 knots. Also it is possible for the greenhouse windows above your head to fall in on you.
HYD is similar to 212/412 the pump is turning faster. Design thing, the second pump came along later and is driven from a different part of the MGB.
When do the secondary nozzles open? Who cares, just that they do.
B1 means reduced internal gross weight over B and is only a flight manual change to be technically under 12,500 lbs.