she already looked worse...not able to stand, wings spread out to balance herself,
Ginene, these are classic signs of eggbinding as the pressure from an unlaid egg can cause severe pressure on the spine effecting their ability to perch and/or even stand. Often when they have wings splayed like that, they are also straining to lay the egg (which may or may not be noticeable). An eggbound hen will usually pass away in a matter of hours if she is unable to lay the egg. I had one shaft tail hen that I had to administer drops about 4-5 times over a matter of 2-3 hours because she was severely eggbound. Did not even think she was going to make it as she was going downhill fast. Thankfully, she finally laid and took a few hours to recover and was fine.
I'm sorry, I'm still suspect of this vet's visual diagnosis of something as extreme as avian pox based on visual only and when only one of your birds has had any sort of lesion. Sally's story about her vet doing the same thing even with a confirmed test that later turned out to be a false reading further makes me wonder. I can respect that you want to listen to the vet's advice but I've heard almost non-stop stories of misdiagnosis from avian vets and very few stories of avian vets actually successfully treating finches (and other birds). With a diagnosis this rare and extreme, if it was me I would certainly demand more proof before I would believe it, and certainly a second opinion. I consider my birds to be members of my family - if my husband or one of my kids received a diagnosis of a terminal illness, I would want more than just one opinion. I do think you are doing all the right things in taking precautions, maintaining a cautious attitude, I would just hate for you to go through all this heartbreak only to find out you were yet ANOTHER person wrongly diagnosed.
Even with people doctors, they can often and do make mistakes. When I was pregnant with my middle son, I had some unexpected bleeding and was taken to the emergency room. They admitted me and one of the obstetricians from my obstetrics group visited me. She told me in no uncertain terms that I was having a late-term miscarriage and it was certain the baby would die. As you can imagine, I was absolutely devastated. That was one of the hardest nights of my life. By the way, my son just turned 24 on June 7th. He's alive and well, no medical problems, in fact he was bumped up TWO grades ahead of his classmates during elementary school and junior high so no mental side effects or anything either. This doctor basically just decided to state her opinion or lack of knowledge as truth and unfortunately, I thought doctor's had so much knowledge they could never be wrong. Found out that was not the case but so glad she was wrong, I have a darling son to prove it.
Like Sally, I had a problem about 2 1/2 years ago, when two of my rccb's just died within an hour of me finding them fluffed. They had a disease I was not familiar with at the time and I had to clean and do all of disinfecting stuff to make sure no one caught the illness. I saved all the flock, except the two rccb's, but I think it was because I found them right away, even though, they died. 
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