Hi Doug.  I am not in your industry but am a retired Dental Hygienist.  I was trained ages ago before all of these crazy viruses came around but I was working in Pediatrics through H1N1.  If you can work in my industry and not get sick, I am pretty sure someone can tour a home and protect both parties.  All that you listed that your realtors are doing is very wise with only one person touching things in the house.  One simple thing you could add is for the Sellers' Realtor to wear gloves. If the Buyer feels more comfortable, they could wear gloves too.   Most recent reports from the CDC is metal doorknobs and wood furniture are the most likely to have the virus remain.  Both are difficult to disinfect completely.