If so, is it legal to wire a 15 amp receptacle into a #12 awg 20 amp circuit in the "backstabbing" manner noted above?
WFO, Receptacles with backstabs are no longer made with holes that will accept wire larger than #14. It's been about 10 years since the ones with #12 holes were discontinued. Also, the receptacles state "#14 Solid Copper Wire Only" right on the back.
