My take / advice is somewhat different.
First, most medical errors are systems problems. Good people make mistakes because the system allows them too. Making the system better is the right answer, and if signage isn't good then that's an easy step to take to try to prevent these sorts of things.
But
This was an external accreditation visit. This isn't just an internal safety officer issue, the entire institution could receive citations over this, and that could be very painful for them. The stakes could be very high. Anything the OP can do to help mitigate that is in everyone's best interest.
The OP mentions that they are not an employee of the institution, but rather an external contractor who works there. The institution could simply say they no longer want this person working at their institution. There would be no right to a "fair hearing", and as long as it's not an illegal discrimination issue there really isn't anything the OP can do. Their actual employer could then assign them somewhere else, or do whatever else they wanted to do.
Most likely, nothing will happen. Blaming other people will almost certainly end up with a bad outcome for the OP. Submitting a complaint, after the fact, will look like retaliation / CYA.
I used to think similarly, and have been convinced otherwise. This has been shown to be inaccurate. Contact precautions are usually not for the physician or nurse -- they are to protect the other patients in the hospital. C Diff transmission is markedly decreased with contact precautions. You won't get it -- we're trying to prevent the next patient you see from getting it. Whether you touch the patient or not doesn't matter -- once the patient has been in the room for awhile, the spores get all over due to the patient moving around, other people touching the patient and then other surfaces, etc. Out general rule of thumb is "swing of the door" -- if you stand in the doorway and talk to people in the room within the swing of the door, then that's fine. Any further into the room than that, and you gown/glove/wash.