Would you be able to see the aorta for aneurysm on a lower back mri?

Yes. The majority of MRI exams of the lower back include the abdominal aorta. Occasionally, the front part of the aorta is cut off (not included in the images). This may necessitate additional imaging such as ct.
May be. May be but not the best test and will miss some aortic portions.
Generally not. Usually the area of the aorta is blocked out of the image so the study focuses on the vertebral bodies, disks and central canal.