I don't know the answer to B or C as I've never looked into OFAing anything but a registered purebred. I'm sure if that information isn't on OFA's website, a call or email to them would answer those questions.
As for A - it is ALWAYS a good idea to know a dog's hip status. Many of the problems associated with HD are due to arthritic change within the joint that develop over time. This can be significantly slowed down by proper diet, exercise and the addition of joint supplements. If the hips are really bad and surgery is needed, the earlier the surgery is done the better the long term prognosis. Mutts and unregistered dogs deserve the same quality of life as purebreds and with the large breeds, and many of the smaller breeds, an important part of that is knowing joint status and intervening for the welfare of the dog when necessary.
As for A - it is ALWAYS a good idea to know a dog's hip status. Many of the problems associated with HD are due to arthritic change within the joint that develop over time. This can be significantly slowed down by proper diet, exercise and the addition of joint supplements. If the hips are really bad and surgery is needed, the earlier the surgery is done the better the long term prognosis. Mutts and unregistered dogs deserve the same quality of life as purebreds and with the large breeds, and many of the smaller breeds, an important part of that is knowing joint status and intervening for the welfare of the dog when necessary.