The shooter still had fingerprints didn’t he ? How about “future” protocol established to handcuff then use his fingerprints to open his phone before he reaches room temperature ?
Body temperature has nothing to do with whether the TouchID works or not. It has to do with other factors including blood flow and body electrical capacitance when changes drastically at death. But the most important is the fluids still being in the fatty pads under the finger tips which are maintained by blood pressure. They'd have to catch it quite quickly for it still to work. These days, disturbing the body in situ at the death scene is not permitted until CSI is finished with it. Also, I've heard no evidence they'd found his iPhone yet. He may have left it at home.
Besides, it still takes a court order to "search" his iPhone.