Colleagues of mine have a support case open regarding the unsupported status of Owner, Pre-Conditions, Validation Input, and Validation Expected Result. There may be others.
In connection with that, we can confirm that the following are all supported: Object Type, Name, Description, Test Folder, Expedite, Objective, Work Product, Type, Method, Priority, Risk, Affected Products, Notes, Input, Expected Result, Step Index, Test Case. But we can't say which are required.
If it wouldn't be right to share your data publicly, at least please let us know the outcome of your support case.
Best,
--Nev.