Taking out Warren won't assure Hillary the nomination. If Hillary and Warren are seriously damaged, someone else will step up to try and claim the nomination. That's how Bill went from a second-tier candidate to the nomination because all the top-tier guys chose not to run. There are plenty of relatively-unknown but wildly ambitious Democrat politicians who would step into the void.
Right.
Brian Schweitzer of Montana, Kirsten Gillibrand, Cory Booker, in addition to more frequently named backups.