SW, my boy has a number of issues, low end platelets is just one. Doxy is his primary abx. He gets better on other abx, but he also has some secondary Staph issues, eye issues, nasal issues, skeletal issues, perianal fistula, the list goes on. His platelets aren't given the opportunity to drop too low - I run regular bloodwork and pulse with antibiotics when they start to get too low. They have only been scary low once with the azithromycin. He doesn't get bloodwork until next week. When my Max was on Baytril, he did improve, but he also developed a delibilitating back problem that I'm still dealing with a year later. I had him on a doxy and low dose baytril regimine, used to treat the ehrlichias and coinfections like Bartonella.
I think what my boy has and what your girl has are probably different to a large degree, or at least manifest in the body differently. The important thing to me is that the blood profile you have is pretty textbook, and I would hate to see a missed opportunity when the stakes are so high.
There is one vet that used to be on the tick list that believes that it takes about 5 yrs for the chronic form of an ehrlichia type disease to develop into a chronic disease that shows in bloodwork. If this is the case, I guess anywhere your girl has been in the last 5 yrs is suspect, and depending on her age, parental exposure is also relevant.