as Talent Manager here at PrepLounge I am happy to provide you with an answer to your questions.
Indeed, the better you have read through the case before giving it to somenone, the better you can guide your interviewee through the case. However, we are trying to design the cases here at PrepLounge in a way that allows the interviewer to give it rather spontaneously to others. Do not worry too much, and if you get to be the interviewer first you can enter the meeting room and familiarize yourself with it and the case (and change it, should you want to). Note that with cases you found online sharing exhibits can be quite clumsy.
Joining a meeting as the interviewee only is theoretically possible, but not the usual thing that we see happening between users. To have a full session of being interviewee only make sure to check out our experts here. You do not have to be the interviewer with them ;-)
Hope this helps, sunny regards from Cologne,