This new bookkeeper seems to be the cream of the crop and I'm paying $500 per month plus 2k set up fee (After having paid my last bookkeeper 4k to clean up my books) (my point is... am I spending enough that they should have a fix for me or can I not rely on that?) 

Their infection rate has dropped low enough to partially re-open their economy due to their extremely harsh reaction. 

Is there a letter or cleaning list you send to the tenant addressing what would need done before they leave?

Honestly, with COVID here and so many people not working; if someone that has a clean background and has a job moves in with a current tenant, it just gives more financial stability to the unit.

I am personally using the opportunity cost as a benchmark for my decision making; how much will it cost in terms of missed rent, costs to clean and market the property vs how much will I lose is I let a few months slide.

How about cleaning the unit/house with a hospital disinfectant? 

lawn mowers & cleaning people still tuff. 

That means gutters, clean gutters, drainage pipe leading away from the house, base pitching away from the house, etc.

If the vapor is coming from the soil, wouldn't the proper way to remedy the situation involve actual soil removal/clean up?

Between renters a cleaning crew can use the key code that’s provided to them and have the unit ready for the next renter.