Less expensive, yes, but but not as safe if what you are worried about is getting struck too hard by the airbag or getting jammed up against the steering wheel. A pillow is going to put you even closer to the steering wheel.
I can adjust the height of my seat, but if I put it as high as it will go I can't reach the pedals properly, though the seatbelt still isn't quite right.
I'm 5' 3.5", and I don't have issues with reaching the pedals correctly with my seat adjusted for good vision, but I am rather close to the wheel normally. I used rubber-covered blocks taped to the pedals when I was in the third trimester; it was a good-enough temporary fix for only 6 weeks or so. I wouldn't do that permanently, however, because I'm sure they would start to get loose fairly quickly with regular use.
I am only 1/2" taller then you and with my seat all the way up I can't have the belt adjuster all the way down or it squishes my shoulder too much. I am able to sit an arm length away from the steering wheel pretty easily. It sounds like pedal extensions would be a better solution then a seat belt adjustment.