In a rental you will not believe the slurry of stuff they grind up, in fact I prefer to use 2" PVC below the sinks to avoid clogged 1.5 & I've had many.

They adapated to the situation, continued to grind, took advantage of opportunities while others cowered in fear, and they were rewarded.Hershey is a great example: "Hire 40 men!" 

Anyone else having issues with tenants who you've recently found to be damaging the property or dealing with several noise complaints?

Keep grinding

Auger bit will grind/pull out any debris.  

Keep grinding.

You are being very generous here, a discount doesn't solve their problem, if you were going to do anything you'd buy noise cancelling headphones, but in reality construction on stairways, roads, landscaping, pools etc. just happens and there isn't anything anyone can do to stop it.

2nd floor apt is complaining about noise from 3rd floor apartment, primarily at night when the above tenant is still active and the lower tenant is sleeping. 

For example, violating noise clause in Rules in Regulations which is part of the lease.

4 buyers- build you buyers list and be in touch with them.get out there and start grinding, dont wait to know it all!