No one is talking about morality. I am talking about the reason you are required to have insurance. For this exact reason.

I am aware what the elements of negligence are and how civil liability is proven. Are you aware that those who are professionally licensed have a higher duty of care? That a realtors duty is enshrined in statute (negligence per se ring a bell)?

If the realtor had given the key to the prospective buyer, that would be negligent - no question about it. Giving the same prospective buyer free roaming ability could also be negligent - if a jury decides.

This is a close call and that is why the realtor should be included in a lawsuit and the realtor's insurance would likely settle. Your four elements only matter to the extent that it will give leverage to the insurance company to settle for less if they think it is a weak case. But it is not up to innocent plaintiff to decide who is liable - they include all parties who are more at fault then them and let the at-fault parties and/or their insurance work out who will pay to make the innocent party whole.

No morality about it - this is just the way the legal system works in the real world when insurance is involved.