I am sorry you're going through this. I know it is very stressful to you. I love my boas so much too, it's hard to even express it. I feel for you, I know you are scared. Until the vet gives you the definitive diagnosis though, try to put it out of your mind, it will only make you more miserable. When you get the diagnosis then you will know more. Maybe the vet is wrong. Wait until you get that info before freaking out. You didn't say how your boa looks. Is your boa showing neurological symptoms like laying all crazy or looking endlessly at the top of the cage?
You need to stop giving your boa the well water though for now. It could be your problem. Have you ever had your well water tested? For now you might get distilled water, and give the boa just pure clean water. Boas don't like drinking water with stuff in it, and your snake could get dehydrated from not liking it's water. Boas don't need calcium they get it from the bones of their prey. Snakes eating rodents don't need vitamins either, everything they need is in the rat. If you insist on giving vitamins, you should inject the prey to get it into your snake, or get spray on vitamins, but don't "dirty" your pets water. Like every living thing boas need pure clen water. Water is more important than even food, certainly more important that vitamins and calcium. What kind of vitamins are you feeding this boa? Most are only chemical fractions of real vitamins which are whole complexes the way nature intended them...in food.
What did your previous boas die from? Is there any possibility that you didn't clean up well enough and you boa was exposed to the same problems your others had? The retrovirus IBD is thankfully like AIDS in that it isn't likely casual contact can infect someone, be it snake or human.
Man, I feel for you. I know you are doing everything you can. Sometimes life hands you bad stuff. All you can really do is try to hang in there, and ask all the questions you have to the vet first. Make your vet unserstand that you have to know the bottom line.
The very best of luck to you,
Caden