This didn't get to the root of the problem, which was a blockage in the main line. 

You inherently will want to please your future mother-in-law and be nice, but that's a horrendous blockage in how you may view the transaction.

Tenant shall be charged for all damage to Premises as a result of failure to report a problem in a timely manner.Tenant shall be charged for repair of drain blockages or stoppages, unless caused by defective plumbing parts or tree rootsinvading sewer lines."

And then get landlord insurance and a flood certification.I'd have a plumber scope the sewer line (blockages, cracks) and look for any nasty weak spots in the house in the plumbing (mold, water soaked walls, dry rot, termites, stains) just so you know what you are getting into.

With the next blockage you might have s strong indicator of the offender. 

As far as augering out your drainage system, you can rent a power auger and do it yourself (or hire someone) but be prepared for it to take several hours if the blockage is significant.

No damage, deformities, blockages or anything abnormal noted during the inspection.

Alternatively, there may be a situation is which the water is pulled out of the p-trap too quickly, siphoned out by inadequate venting or movement in the drainpipe, or some kind of blockage, or, or, or.Please don't tell me that not one of your four plumbers suggested this possibility.

Not sure what it is but I have a plumber going tomorrow  Probably a blockage in the main line.

This way you can seek specific help on blockages you have instead of just asking them to give it all to you.