"Slow" plumbing

[Karyn T.]

Our rental property is a very well kept, historic home, built in 1924, located outside of Seattle.  We moved our current tenants in October 2014.  Within 4 days, they (the wife, in particular) had caused $12K in plumbing damage by pouring draino down the 100 year old pipes.  The trap under the master tub let go (I realize it was likely coincidental, but likely hastened by the heavy chemicals), the water then ran down 2 stories into the basement, taking out 2 "vintage" plaster ceilings, basement carpet, etc.  We had it all repaired and a week after the final payment to the plumber, the hot water heater let go (which had been inspected prior to rental).  It was replaced with new as well.  The entire system was scoped at that point, and found to be clear.  We got a home warranty at that point....

In January 2015, there was another stoppage.  "Sand" and a child's toy was found (she claims her boys are "too old" to have done this...8yo).  We had the system cleaned out again.

Two weeks ago, she notified me that "all 3" of the toilets and sinks are "running slow" and wants the system checked again.  The plumber came out and determined that the basement toilet has a "hairline crack" and needs to be replaced.  I was told by the home warranty company that the plumbing report came back as a "hair clog" in the master bath sink.  

Today I get an email from the tenant stating that the toilet repair was approved by the HWC, but not additional scope work, which the plumbing company wants to perform.  They are concerned there may be blockages in the "Main Line".  

Because of times zone issues, I have to wait until tomorrow to talk to the plumber, and I'm currently on a 20+ minute hold with the HWC.  Assuming they won't cover anything but routine snaking (which seems to be covered in my warranty) am I responsible for issues in the "main line" (ie: outside the house).  The city forced the entire neighborhood to put in back-flow valves last year at the same time we were getting all of the repairs done, because they were replacing the 100+ year sewer lines that were already known to back up into the houses (causing a few lawsuits against the city).  It is my understanding that the lines past my back-flow valve are "not my problem".

Also, what is my responsibility for "slow drains"?  Where does it end?  When is good enough, good enough?

Thanks in advance.  

PS - this is a ~4000sqft, "high-end" rental.  It is NOT a typical investment property.  It was our primary residence for 2.5 yrs with ZERO plumbing issues.  I know old houses need maintence, but with a clean bill of plumbing health last fall, this is getting ridiculous.