1 the opt as specified by the fmc. why, cos the book says its optimum. you dont have to know the particulars.
2 go back to q1. do what the fmc suggests. who cares why you dont fly as high as possible
3 who cares. cant control it.
4 who cares. call him up and ask him. ask atc. watch him on tcas.
5. cos the fmc told you to. not rocket science ya know.