What a sad situation here. I don't want to get into all the drama but there's a few medical questions in here I want to address from my personal experience. I'm NOT a vet, but I've been in the field for 8 years, have lots of experience with necropsy, pathology reports, and sick animals. I'll help answer some questions in here.
Pneumonia has many causes and a lot of them can stay "hidden" until the animal is stressed as stated somewhere above. I think this is the most likely cause here. This doesn't make anyone in this situation guilty of killing the snake, it's just an unfortunately thing that happens. Animals get sick, and they die. I see it every day at work. It's very sad and emotions do not help in coming to a conclusion that is scientifically explainable.
While viral infections seem to hide easier than bacterial ones, natural flora CAN cause infections when (again) the animal is stressed etc etc (think Salmonella!) so that part really isn't indicative of when the snake caught the disease. The necropsy report also may not have time frames, be prepared for it NOT to say how long this snake was sick. Most necropsy reports contain details about what shape the organs/body are in, and possibly why (post mortem changes or caused by disease?). It's really hard, even for a veterinarian to tell how long a disease process could take to kill an animal. Pathology may also include gram stains and possible organisms or cysts found in tissues when looked at under the micrcoscope. If it's not clear, I would take that report to your DVM to interpret for you.
Necropsy can be expensive depending on where you go and here's why:
If your vet is the one doing the GROSS necropsy, they have to dissect your animals and decide which organs to send for the histopath (this may be because they either LOOK suspicious or they are allowed to send a certain number of tissues so they pick the major organs). If the vet sends a liver or a kidney it doesn't mean they THINK the animal died from liver or kidney disease. The lab usually charges per tissue, in turn the vet charges you that PLUS the charge for their time for dissecting the body, packaging, time etc. This probably does not correlate to an ultrasound charge at all. That's totally an "apples to oranges" comparison there. I've worked at places that charge way more for an ultrasound than a necropsy, hands down. It all depends on the vet who does it, if they are sending tissues, or the whole carcass, handling fees, etc. (Most places I've worked do IN house necropsy and histopath for tissues) and they don't charge by size of animal. The bigger they are the EASIER the necropsy is, not vice versa!
The other thing is, the snake in the photo doesn't look dehydrated to me, look at the mucous membranes in the mouth- MOIST! You couldn't convince me that's a dried out snake unless I touched it myself. It looks like a freshly dead, urate covered snake. The blood in the mouth is still red, not brown or black. I've seen many snakes like this. I've also seen slime coming out of them, like in the other photo. If it was from the nose, it would have the possibility of being URI, or URI with pnemonia. URI can also move into the lungs though, causing pnemonia. Once it's in your lungs, (and im talking all species now) it's serious. Not only is the infection spreading like wildfire and having direct access to the bloodstream through the lungs, but you also have all this mucous spewing out the poor snakes little trachea making it hard to breathe. It's usually pretty OBVIOUS the different between URI and pnemonia. I've gotten to the point where I do oral exams on all the snakes that come in my house. I check for mouth rot and clear airways! Pnemonia is difficult to cure once it's set in. There's the dehydration factor, problems breathing, disease itself spreading and causing sepsis, and the fact that reptiles don't handle the good antibiotics very well. My point is even if it was caught a day earlier, or even a few days earlier and treated, it still may not have survived. I'm sorry for your loss- both seller and buyer.
If anyone has any other questions about necropsies or anything feel free to ask...I hope I answered some of those lingering questions.