Personally, I think if you go with the one gallon, the plant is normally larger and its roots are usually already well established. I really don't think you need to pinch or trim.

It depends on how tall the trellis is. Since you intend to confine it in a certain area, you need to guide the new growth by using plant ties or manually guide in the desired direction as this plant is not naturally a climber.

Star jasmine grows pretty fast and is vigorous. How big is the trellis?