Thanks, TWA. I found the following on a site with consolidated UMNR policies by airline dated mid-2017. None of the info I’ve found about Delta seems to indicate a prohibition on connecting flights, as it does (in a very obvious way) with the other big US carriers.
Age: 5 to 7 years old – nonstop or direct flights only, 8 to 14 years old – nonstop and connecting flights; Children under 5 cannot travel alone.
Cost: US $150 each way.
Flight restrictions: Unaccompanied minors are not permitted on Red Eye flights (between 9 p.m. and 5 a.m.), with the exception of flights departing from Alaska or Hawaii and connections to/from international flights.