I am sorry to hear you are worried about your Morkie. I am a veterinarian and will do my best to help.
A low platelet count like 18, if it has been double checked to be the actual reading and not a lab error, is most likely due to IMT, immune mediated thrombocytopenia, and prednisone is the best treatment. If this is what she has, her counts should start going up in a few days, and you start to very gradually lower the dose of prednisone. If the counts stay low, I would start another medication such as cyclosporine, or azothioprine.
Another cause of low platelets is an infection by a tick borne disease like Lyme disease or Ehrlichiosis. The treatment for that is doxycycline, so your vet is doing exactly what I would do for my own dog.
That is a normal BUN, and normal phosphatase level, so I am not worried about her kidneys and liver based on that.
That is a normal neutophil and monocyte count as well.
I would be checking her soon to see if the platelet count is going up, or not. Let me know how she is doing and what else I can help with or answer.