20A and 50A receptacle on 30A circuit
Hard "no".  The only receptacle you can install on a 30A circuit is a 30A receptacle.  NEC 210.21B3.   



You mentioned the EV industry's tendency to put 14-50 on every charger.  That creates a real mess.  You cannot install a 50A receptacle unless you have 50A wire and breaker.  As you note in the above table, there's an exception where it's also allowed on 40A circuits, but the reason for the exception is there's no such thing as a 40A receptacle, and that exception is bullseyed at the special case of range/ovens.  For a new -50 in a garage, where anything could be plugged into it from a better EV supply to a welder, best to use 50A cable so people get exactly what is on the tin. 

Contemplating a situation where replacing the 10AWG cable is not practicable... I would get a "generator transfer switch" that takes the form of a small subpanel with an interlock so two specific 240V breakers can't be on at once, and takes readily available breakers (because I'd change them). Normally, those panels backfeed; I'd not.  The 30A supply would go to its buses.  I'd change one breaker to a 30A and feed the 14-30.  The other to 20A and feed the 20A receptacle. 



If forced to plug into 30A service, I would use a 30A receptacle and a cheater cable.  This is not quite legal, but since it's not part of the building, it's out-of-scope for the permitting and inspection process. I'd specifically aim to make it as obvious as possible that it is a cheater cable -- to warn "the next guy" that this is not 50A as advertised.  A receptacle installed in a building should always be what it looks like.  I shudder to think of people solving this problem with a neat 14-50 installation onto 10AWG wire; later they sell the house to a guy motivated by that sweet 50A service in the garage.