This isn’t about details: these are big mistakes! Preparing recruits for a research session is a shared responsibility between the recruiting agency and the researchers, and it sounds like there was lack of alignment on these responsibilities (as well as errors in execution, especially by the agency).
Questions from the recruit like the ones you mention (“Why were we asking these questions? Who do we represent? How are we going to use this information?”) should be covered by your introduction call and the initial conversation during the visit, even if the recruiting agency already covered those in their selection process. Some agencies say they do this but “forget” they only do it when people sign up, and don’t repeat it at the point of selection for a study. That’s why I make sure to always “repeat” the relevant information at the start of the research session. I have even decided to highlight these bits in the NDA/consent form, which nobody reads in all of its details, but at least the relevant bits stand out this way.