I haven't experience of running a wall, but used to advise sports facilities on liability issues.
I would just give your insurers a call. They will either cover you to run the activities or insist on having qualified people in place.
Sometimes they will insist that you do the risk assessment yourself to decide whether you have the right kind of experience/skills in-house to deliver the sessions. I've seen people change their approach/opinion after doing this kind of assessment, and it's a not a bad start point to go through the issues systematically (not saying you haven't already done this - attitudes and approaches vary).
Having seen this scenario play out dozens of times, I think you're attitude is right about the guy who walked out of the arrangement. He will probably not keep up the sessions, and in any case you will have figured out a way to deliver something better quite soon.
Can you not find a qualified trainer with a bit of climbing knowledge to help you out?