It has so much potential for additional costs...Sort of devil's advocate, what's the harm in letting them go for free? 

The flip side to that being it's not what I would say is necessary for a non-luxury transaction to be taken seriously- actually harmful in these cases at booking time it seems. 

Instead of wasting your money you might want to print out the harmful effect of not replacing the filter for residents and make sure the tenants always get a copy and warning,     Also you can take it a step further and you can-write it in the lease that "if the filters are not replaced regularly tenant understand that this will cause a $300 repair bill at the end of tenancy, where tech or HVAC cleaning co. needs to come out and clean the system."

They have the most vested interest in you buying a solid deal, because they have to manage it every day.Just be careful of turnkey operators as they have a bigger vested interest in selling than managing.You might want to read our series about “How to Screen a PMC Better than a Tenant”, since selecting the wrong PMC is usually more harmful than selecting a bad tenant: https://www.biggerpockets.com/member-blogs/3094/91877-how-to-screen-a-pmc-better-than-a-tenant-part-1-services-and-processes

I'm not an accountant, and this isn't advice, but as I said, you're asking the right questions and there's no harm in trying a new accountant next year.Best of luck!

Failure to do all of this could result in you being liable should someone actually be harmed. 

Yes, as usually happens, laws designed to "protect" people (eg discrimination) end up indirectly harming those they meant to protect. 

Before you hire a PMC you might want to read our series about “How to Screen a PMC Better than a Tenant”, since selecting the wrong PMC is usually more harmful than selecting a bad tenant: https://www.biggerpockets.com/member-blogs/3094/91877-how-to-screen-a-pmc-better-than-a-tenant-part-1-services-and-processes

The vast, overwhelming majority of people are not out to harm anyone or burn your house down!

In situations like those, I know the best thing to do, is send out mailers to see if homeowners would like to sell their homes, and see if I can make a cash deal, no commissions needing to be paid or closing costs.The issue I run into is that I am now a licensed agent in California, and I don't know the best way to handle looking into self investing options, when as an agent, you are bound to make sure homeowners are not harmed financially in the sale.