Sorry to hear that your dog is having problems!
This is indeed a high white cell count! It does sound like your vet is doing all the same things that I would.
I likely won't be able to say for sure what is going on with your dog, but I can give you a few thoughts.
If your dog is acting normal and eating well, it is possible that this is a response to stress. Some dogs can get so excited when they go to the vet's that the white cell count increases. Granted, it is not usually as high as this, but it is theoretically possible, and even more so if there is no fever. You may want to see if your vet could send a technician to your house to draw a blood sample to see if the white cell count is just as high when at home.
I just did a search for you on Veterinary Information Network where vets can talk about interesting cases. Most of the vets who posted questions about cases like this ended up chalking the increased WBC up to stress. However, the next suggestion is to do all of the things that your vet has already done.
As you likely know, an increased white cell count usually makes us worried about either infection or cancer. If antibiotics did not make a dent it is not likely infection. It sounds like the pathologist has ruled out bone marrow cancer. Cancer in another organ is possible. If not already done, an ultrasound is a good idea. Sometimes this is able to pick up cancers that the x-rays and scope could not.
However, if your dog is otherwise normal, this very well could be a response to stress/excitement.
I hope she does ok!
I hope this information helps! If you have more questions about this concern, just hit reply and I will respond as soon as I can.