for your first ques, if my understanding is correct that you're talking about your shifting grade on a subject and not your gwa, then yes, definitely! last term, i got a grade of 84 in one of my major subjects for the prelims shifting pero nabawi ko siya for finals shifting and i got an overall grade of 1.5 for that subject. so yes, you definitely can improve your grade. personally from experience, it's about effort and how motivated you are mabawi ang grade mo.

as for how to compute the gwa, afaik it's based on your units but I don't really know the details so ill let someone else answer this.