They used a plunger and got rid of the blockage. 

How did you find these properties, don't you have another one in Vegas with a sewer line blockage 20 feet below and city wanted you to dig it up and tenants had to relocate?

If the floor drain is backing up, you have a blockage in the line somewhere which could be anything from a toy that is stuck to tree roots to a frozen valve.

In a SFR, there is only the one tenant, so it seems appropriate there to blame that one tenant for the drain blockage. 

Yes you can run an aluminum (chimney) liner down the chimney (assuming it fits & there is no blockages) & hook the hot water tank exhausts to it.

While disposals do make the solids smaller in size, the material still goes down the pipe and gets trapped in the defects causing blockage sooner and more frequently.

If water is rising in the tub, the blockage is below that point, and any water run into the drain stack above that point (such as a second floor bathroom) will raise the level in the tub.

Collapses and blockages in sewer lines lead to overflows into nearby streams and basement backups.

For example, if the main drain has a blockage and filled with water, and someone unknowingly flushes the toilet, the positive pressure created by the flushing action had no where to go due to the blockage, in a conventional atmospheric vent the positive pressure can just be relieved, but not with an AAV.In some jurisdictions, they allow you to upsize the drain line in order to allow part of the drain to serve as a vent.Also, try to explore an option where a conventional vent can be implemented in the island, but creatively disguised as a support beam, a decorative pole or something.

Cash blockage?