Here are your answers
1: Halberd of the rock isn't part of a quest, it is a strong weapon but it drains your stamina even if you carry it.
2: If you save skill points, then you may choose more later, e.g.: If you had nothing you want on level 18, save the skill point, and if there were 2 skills you wanted on level 19 you could get both.
3: Captain Mitox sometimes goes into a building directly north of his camp, see if he is there.
I hope this helps.
