If you have old cast iron or galvanised drain lines, you will also have more problems with low-flow toilets as the inside surfaces of these pipes are more likely to not be smooth - or have partial blockages.

This can be rectified with the adjustment of pilot flame, unless it is caused by blockages in the burner tube orifices.

If the blockage had been between the toilet in one unit and the main line, then you could narrow down who might have done it.  

He says we did it right and can't see any blockage of interior or exterior footer drains to sump pump that could cause seeping.

Contact the plumber and find out where the blockage was found in the system.

The only time I could see it being charged to the tenant is if they caused a blockage or if the system is filling up unnecessarily fast.

I recently had a simple drain blockage and called Roto Rooter.

As it was, though, we thought it a much better idea to try the simple solutions ourselves first, in case they worked, before calling in the experts when the average main sewer blockage repair bill was anywhere from $400 to $900, depending on the website researched, and I doubt that includes the cost of clean-up of the crud after the repair, in any case.  

The plumber was called in and removed the blockage and the restoration company has provided fans, dehumidifiers, and microbial compound to kill the bacteria associated with the overflowing fluids.My downstairs floor is laminate wood flooring.

It would turn "open" at times and turn "closed" at times, thus causing the intermittent blockage.