Hello and welcome to the Discussion Groups!
It looks like you've done some research, and by the looks of things, submitting the problem to Autodesk and opening a Support Case would be the proper course of action.
If you'd like us to investigate, we could use a sample project file - open your Project file (Detach from Central if Workshared), delete all elements/unnecessary Views/Schedules/etc., Purge Unused as much as you can, manually delete all MEP Fitting Families through the Project Browser, save it as a separate Project. That should get the file small enough to post here.
**Note that some users have encountered difficulties with attachments, so I recommend first posting to the Test Board to make sure the attachments are working properly for you.
You have to be on subscription. Go to Autodesk.com » Support ▼ Autodesk Subscription, and sign in to the Subscription Center. Once logged in, you will see "Technical Support" towards the bottom of the page, with the option "Create a Support Case". I don't believe that you will be able to attach the file initially, but I'm sure that they will contact you and provide a means to transfer it once the case gets going.