Multiple TPs referring to one area
In my practice, I have many similar experiences with clients who when working seemingly unrelated parts of the body (but we know they're all related!) there are referral issues. Mostly it is due to tightness in the fascia itself and not a specific muscle. If you haven't already, you might want to check out Thomas Myers Anatomy Trains books. If you aren't squeamish, you would also like his dissection video where he and a prosector ( a person who prepares cadavers for dissection) dissect and show the fascial lines in the body. These lines make up our muscular system and Thomas Myers has done phenomenal work showing how they connect and function in the body. Somehow watching the video really made the lines (drawn in the book) seem more real.
Anyway, if you aren't familiar with the work, one of the lines of fascia with the muscles is the spiral line, this includes the rhomboids, the abdominal aponeurosis to the opposite iliac crest. Then, also there is the front functional line which includes the shaft of the humerus, latissimus dorsi, and runs in the opposite leg to the adductors. The scalenes are part of the lateral line starting at the occipital ridge and coming down the lateral side of the body (excluding the arms). If you think about these fascial lines, the referrals your client gets into her armpit and back make more sense. Her mild scoliosis, if not structural and may be created by or is contributing to the tension in these lines. What would be interesting is to discover how those areas have become shortened in her life, what types of physical activities she currently does and used to do, how she moves during the day. Those are all clues that you can learn to help your client from regaining those issues when you have helped them to release.
Any myofascial or other modalities can help release that tension, active engagement type work to the areas that refer with awareness of where the lines are located and how they work is a great way to start. Another thing to keep in mind is that pain doesn't usually signify an area that needs work, so although she gets spasms in her subscap and rhomboids, that probably isn't where the issue is and, as you've already found in your sessions, once you relieve the real culprits her symptoms will improve. And it's easy to forget when it comes to our work that often when we help someone with a tension issue, the problems will return if the client doesn't change whatever movement patterns or habits that created the issue in the first place.