Can I apply for a grade withdrawal during a leave of absence?

No, you cannot.  You can only apply for it when you are enrolled.

I got an even lower grade than the original.  Can I make a choice between the original grade and the grade from retaking the course?

No, you cannot.  Personal selection of grades can undermine the stability and equality of the grading system.  The grade from retaking a course becomes your final grade.

I would like to get a partial tuition fee refund for the withdrawn course.

Since the course withdrawal is made through individual student decision for personal reasons despite the course being offered normally, partial tuition refund is not possible.