[BobO]

Was there any monitoring of the building for movement during this time period?

As defined by the warranty, 1" in a 20' span is the trigger point for a claim. If you have that much movement it will be blatantly obvious. Buildings tear themselves to pieces at less than half that.

My advise would be this; pay your plumber to install a flexible joint at the pipe that broke. Watch it on a schedule and note any changes in a log, continued differential movement will be apparent without breaking the plastic pipe.

You can pursue the original contractors, but, without definitive proof the only people who are gonna benefit will be the lawyers.