Is yoga likely to help or worsen my back pain?

It can do both. If you have back pain, some yoga postions will stretch you beyond your level of comfort. You may have shortened painful muscles as the cause of your back pain. You may need treatment to relax and lengthen your muscles and reduce your pain before yoga can be successfully used to enhance your conditioning.
Depends. It depends on the style of yoga you practice. If you have back pain already, you should let your teacher know, and practice a gentle/restorative style of yoga.