I have an old 15 clone that loves to do free motion work. I also love using a knee controller but it takes some getting used to, I did a lot of sewing with a knee controller until I was used to it and could use it without actively thinking about it. You might think about basting your layers better to control the puckering on the back. Sometimes I'll use a combination of pins and thread basting when I'm gonna be shoving and bunching a larger quilt through the machine so my back doesn't shift on me. If this is your first attempt with the 15-91 I'd say just practice some more until you're real comfortable with the machine.
Cari