Forum rules prevent me from posting the meetups but if you PM me I can send you a list of 4-6 that I like the most.

There are segregation of duties between real estate agent, lawyer, and property management What else should I do to prevent any chance of fraud or be taken advantage of as a long distance investor?

.- First and most common issue is some seepage around corners of house coming from the downspouts, poor grading, or a clogged or damaged gutter that's overflowing near the foundation wall.

and wind up with them calling plumbers for every clogged toilet or dripping faucet?

What are the risks that would prevent you from achieving those goals.

Chalk it up to experience (tenant estoppels might have prevented this situation), and spend your energy managing and improving the property rather than chasing the previous owner in court.Depending on the remaining lease terms, you could wait until the leases renew and make the change then. 

Of course checking IDs etc... is always the best prevention.

Are there any covenants on the piece of land that would prevent this course of action?

Something that clearly said "both units to be vacant upon sale" would have prevented this from happening.

I had a similar case with tenants and fleas, the best way to prevent this is a good contract strictly stating that you will do the inspections for pests but they must pay if there needs to be any action taken.As far as your situation goes, has this tenant always paid on time and has no other problems?