User rights okay in BOTH files (interface and data)?
Well I checked again and everything looks just fine.
But ended up answering my option when looking at another option to tweek
When editing the Priviledge Set, at the bottom right corner there is a drop-down for "Available Menu Commands" with 3 options, All, Editing Only, Minimum. To tell you the truth I had this set-up to Minimum because I had no clue what it was doing. Now I know! Aparently the All and Editing Only options suffice to keep the options of the container field open to the user.
I hope this helps someone else too!