I think you're unfairly putting too much emphasis on RJs fourth-quarter defense. You don't give up 42 points in a quarter and blame it on one or two players. The team defense was horrendous...
Nobody was closing out on three-point shooters, rotations were slow, defenders were getting picked and switching too easily, there was no effort on the boards...I could go on, but the bottom line is that it was just a downright failure by all involved.
It was just unfortunate that the Mavs' two best shooters got hot at the same time.