They never fixed the error on DB Manager, but they had us login and download the newest QB product directly on each computer and somehow that ended up resolving the multi-user problems.

Initially we were downloading it on a server and accessing/running the install from the individual workstations (like any other program in the history of ever) but QB support claimed we had to sign-in to the camps/portal on each user's workstation, get a fresh download, and install using that. 

For w/e reason it did ultimately fix the multi-user mode. Idk if they pushed a new version between the last time we tried it or what, but by this point I was long past my witts'-end and just took whatever worked to make the client happy.