A Mediterranean couple sought in connection with Williams' death is now considered a "red herring" after further police inquiries.
Organisers of the ladies fashion course Williams' enrolled on in secret will give statements to the inquest too.
The lawyer for MI6 says calling corporate witnesses will "paint a picture and set the scene".
Two experts have tried to reconstruct how Mr Williams could have got into the bag and locked it - The coroner says he wants a live demonstration in court.
The issue of whether or not Mr Williams could have got into the bag and padlocked it from the inside "goes to heart" of inquest the coroner says.
A pre-inquest review into the death of MI6 and GCHQ code breaker Gareth Williams, who was found dead in a holdall in his London flat in August 2010, is deciding on witnesses to call.
They will include Gareth's sister and the police officers who opened the front door and discovered the padlocked bag containing his body.
DCI Jackie Sebire who led investigation will also give evidence and will explain photos taken inside the flat and CCTV of Williams's movements.
An officer from the counter terrorism command will also attend the inquest to explain its role in the police after Williams' family asked for it.
Evidence will also be given about the contents of Williams's locker at GCHQ, which several people had access to.