The Hoover Institution, at Stanford University, has a lot of materials on the Lusitania, including the transcripts of proceedings before Judge Mayer. Mayer is the federal judge, in New York, who presided over part of the Titanic inquiry, and all of the one for Lusitania. I believe, also, that the collection has materials related to Mersey's inquiry. One of these days, I intend to go out there and look through it.
When the British Public Records office reprinted the Brit Titanic Inquiry a few years ago, they mentioned in the literature accompanying the book that they planned on releasing the Lusitania inquiry in book form as well- I wonder if they are still planning to do this? How many passengers and crew were called to testify?
I really can't see the point in the PRO releasing the Lusitania Inquiry in book form. It's not an in-depth (or even interesting) inquiry. The hearing ran less than a week. As few witnesses as possible were called to testify. Because Britain was at war, some of the more sensitive testimony, which consisted mostly of the Admiralty orders issued to Captain Turner and how and why he followed them the way he did, was held "in camera" and was not for public consumption. (This "in camera" testimony was not made public until 1919.)
It was by no means a probing inquiry like the two held after the loss of Titanic three years earlier. The American Titanic Inquiry ran for nearly 1,200 pages and was conducted over 18 days. The British Inquiry was even more extensive, running 36 days during which more than 25,000 questions were asked. By comparison, the inquiry into the loss of Lusitania runs just about 100 pages (including title pages and the index of witnesses), and only 2,303 questions were posed. In fact, one of the surviving firemen was only asked nine questions when he was on the stand.
If my quick count is accurate, only 14 passengers and 17 crew were called to testify.