some answers:
2. Yes 15-91s are cabinet machines. It's my guess that more were in cabinets than were portable.
3. I'm not sure what you mean by check spring, but there isn't a spring that I've ever seen in the bobbin case.
4. $100. is not a great price for a portable 15-91, but it's reasonable.
It's a great machine and is supposed to be really good for Free Motion Quilting. I haven't tried it yet, but have it on my "to do" list.
bkay