I'm pretty sure that downloading the base auditorium, formal dance auditorium, career day auditorium, and graduation auditorium separately is how it's supposed to work. They are separate lot types, and you can have multiple of each and swap them out. For example, if I wanted career day to always stay as Maxis made it, but have different spring/summer/fall/winter formal dances, and use a graduation auditorium from the gallery, I could.
For the thumbnails being wrong, have you deleted your localthumbnailcache and avatarcache from your sims 4 documents folder? They store old images (and other things), and could be responsible for showing the wrong thumbnail.
