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?

Are there any rules or laws that prevent an agent from sending me a property, and then I wholesale it?

Either way, a lot of this likely could’ve been prevented with good Property Manager up front.

Not just for the extra capital, but so I don't have an existing mortgage preventing me from getting into a mortgage on an investment property.I currently rent a home and am completely debt free.  

Wondering if anyone does yearly preventive Maintenance checks at their properties?

If a tenant flushed baby wipes and clogs the toilet, no problem (in fact they should be paying for the whole thing).