If my translation is correct and my knowledge up to date, then its about the exciting current for the coils. If no grid power, your async generator will not produce electric. Only if you have a selfexiting sytem or a selfmagnetizing coil, then a automated system to cut the connection is needed.
a: However, the less then 3 % slip rule, lessens the efficiency or the power output for the generator.
b: The correct speed governor is the weak link and thus going to be your focal point, hence join the club.
c: I think Matt is the closest to perfection regarding governing and controlling, the market available controls are not realy suited for woodgas use and lack the ability to maintain the governed speed with gasfluctuations as is the case with fresh woodgas.