Mersey's son, Clive Bigham was appointed the secretary to the court, but that merely means that he organised it. As in the USA, 'secretary' doesn't always mean a pen pusher. Clive Bigham was a brilliant multi-lingual scholar and his appointment was not just nepotism at work.
McKinstry did most of the work, but he could not have done it all, as on one evening several senators took evidence separately. The interview with Fred Barrett is in a different layout from the rest, so maybe that was also the work of some other stenographer. I wonder if the men you mention were responsible for the general paperwork of the committee.