Here's the way I read the second email: He said
normally you would diganose first before treating, but this isn't normal as you have a 4 day wait for the lab and who knows when for the probe.
He's suggesting that you can try the eye drops for the diabetes as you would see results within a day or so according to Gill. He also did say that there was no test for it, so therefore, if the drops were to give results--that would be the diagnosis. I am assuming since he suggested trying the treatment on her that it will do her no harm if she does not have diabetes insipidus.
I would also think that if she shows a response to the diabetes drops, you may be able to forego the scan. The results would be back from the lab by then and you and he can discuss whether a scan or any other treatments or tests would be necessary.
I'd give it a go if I were in your shoes
