One has booked our house for their kids graduation open house, so they don’t have to host it at their house (let our cleaning crew do the cleanup!). 

Make sure your leases going forward have that no-smoking in unit policy and identify the clean up costs for smoking in a non-smoking unit.

Not saying your friends are messy, but a mechanic shop might have a bit of cleanup involved.You might consider building it, getting it fully rented, then selling it.

Equity stripping with false liens or intent to deplete assets will get you in deeper trouble.

If they are both gone, I would inspect the apartment, clean/repair, and then charge them for rent until the place has new tenants or the deposit is depleted, whichever is first.

I would require them to clean up the feces though as it causes a huge health risk/liability. 

We usually beat out the competition by making the transition painless & close cash asap & we would handle any clean-up, storage of possessions etc that they had not had time to go through. 

Clueless governments over time have added rules to "help" tenants for political reasons such as rent control, then more rules to avoid tenants abusing previous rules leaving a relatively complex system that no new government has the guts to really cleanup (BTW the city of Seattle is trying to go down the same path, clueless politics know no borders).Now, if there are still millions of RE investors, they must make some money somehow otherwise they would all park their money abroad or in commercial RE, leading to residential prices dropping. 

We do the lawn maintenance ourselves for our multifamiliy properties (2-4 units)  It takes me 1-1.5 hours every week for all the lawns and I spend about the same on clean ups and shrub trimming twice per year on each property.Our city DOES have a lawn maintenance ordinance, no weeds/grass can be taller than 10". 

I.E. lawn will be mowed weekly, mulch will be spread once per year, and we will pay for a spring cleanup, etc..