Your description fits for it.

Of course, if the thymus was not enlarged and the chest fluids were
analyzed with actual bacteria, then that would not be the case, ditto
if necropsy showed a major heart or (more rare) lung
disorder/deformation.

It really reads like one of the common presentations of Juvenile
Lymphoma, though. JL, which usually occurs in the first two years of
life, is not very common and usually kills very rapidly. There is no
treatment that works for it. Not all experienced vets have encountered
it, so it does not come to mind readily. Also, it is a rapid death
sentence so vets tend to hope that it is not that, but instead
something treatable. Both ferrets we have had with JL in about 32 or
33 years with ferrets in our family had ferret-experienced vets who
had not encountered JL before. Some of the kits with JL last weeks but
ones dying within just a few days of first symptoms also happens; we
have had both situations.

I apologize but I had to stop reading the FML at that point so did not
finish your post but wanted to write this before going offline to tend
to other needs.

[Posted in FML 8161]