To be totally honest, and it's probably not what you want to hear, but I very much feel that putting her down would be the kindest option for her. If she already has a fractured splint bone, then a fractured hock on top of that, the recovering is going to be slow, painful and stressful for all involved. It will be a long period of box rest, and because there are two significant injuries involved, meaning the horse will want to be taking weight off the affected limbs, the chances of her developing laminitis in the 'sound' legs is very high. Not only that but mentally it is very hard for a horse to be boxed up for such a long period of time.
I put my horse down for a similar reason, though not a break. He was only 10, and had arthritis in his hock, and then went and tore his suspensory and ruptured a tendon in the same front leg. After 2 weeks I couldn't put him through it anymore and elected to have him put to sleep. It was the best choice I have made in a very long time.
After I laid Hugo to rest, I put a post up here about 'when enough is enough', in relation to when to make that final call. Your mare is 17 years old, putting her through surgery alone is a risk, the hock is a very problematic joint, and as the vet said, the chances of her coming sound again even with it pinned are slimmed. Plus arthritis and a fractured split bone.
Personally, I could never put my horse through all of that, because I couldn't stomach saying goodbye. The horse knows no different, it's there one second, gone the next. It does not think about what it's going to do tomorrow or what its goals in life are. It knows only 'now', and if 'now' is uncomfortable, painful, stressful etc. and we as humans have the knowledge that this is going to continue for a long period of time with little chance of a full recovery, then I think it is our duty as the carer of that animal to end their suffering.
Best of luck in your decision, it is never easy.