Thanks for the detailed answers!
Unless the chiropractor was also a vet, I would have a vet examine the horse to confirm the diagnosis by the chiropractor. Chiropractors are very good at what they do, and there are many conditions that chiropractors know more about than vets (e.g. subluxations) but pulled muscles fall into the category that vets know more about than chiropractors.
This is a very unusual situation, and if this were my horse, I'd give strong consideration to taking the horse to the University of Sydney's University Veterinary Centre on the Camden campus, which is located atXXXXX Camden, NSW. See:http://www.vetsci.usyd.edu.au/veterinary_services/camden/clinic.shtml
If she does have a pulled muscle, they will have the equipment necessary to determine how extensive the injury is, and then give you an estimate of how long it will take to heal, AND tell you what you need to do to help it heal correctly. Depending on what they find when they examine her they may recommend xrays, ultrasound, and perhaps even MRIs. Once you have a definitive diagnosis, it may be that R&R turned out in a paddock is just what she needs, but it may also be that she should have some physical therapy, or be fed not on the ground, or some other similar change made to accommodate her injury and help it heal correctly. Without a comprehensive workup and definitive diagnosis you are really just guessing about the nature of the injury and can't make any predictions about what the best care is, or how long it will take to heal.
I would also be sure to have a vet examine her if you have any thoughts about holding the seller and the seller's daughter responsible for the change in your mare's condition between the time you purchased her, and when they actually put the horse into your possession. The sooner you have this vet exam done, the better.