1- I'd use 'for'. A & B would need his/him for me.
2 No; it's correct- the handicap has not been an impediment to the dreams coming true but has, in fact, been the force that made them come true.
3 For me it's b), meaning that they can't be killed by circumstance, but they can die inside.
4 if...happens/should ... happen- a nice sneaky piece of testing there
5 It is a noun, but it doesn't have to be plural- it an uncountable use, like all impossible things, but roled into a single package.
Student or Learner