A: Would he have been able to defeat Vadar without using the Dark Side?
B: Did he indeed use the Dark Side to defeat Vadar. (Yes this was confirmed)
C: Why is it okay for a Jedi to use the Dark Side of the Force to obtain his goal as long as he uses the light side at the end.
A: No, he wouldn't have been able to defeat Vader without the Dark Side. Or even if he would, he didn't intend to fight him. He would have let himself get killed.
B: Yes, it was confirmed.
C: No, it is not okay for a Jedi to use the Dark Side of the Force to obtain his goal. But Luke didn't obtain his goal. After he had beaten Vader, his situation was not different from before. The Emperor tried to lure him to the Dark Side.
His goal was to resist the dark side and to redeem his father. Non of these was accomplished by defeating Vader.