Could we be reading this wrong? We're assuming sacrifice means death (of the Captain and crew) and destruction (loss of the ship).
But if a Captain and his crew willingly violated the Prime Directive in terms of going out of their way to alter the development of a society then if prosecuted it would certainly mean they sacrificed their careers and their ship.
The Proconsul believes death is what the PD means, but maybe he has it wrong and Kirk, Spock and the rest (including Merrick) just let him go on believing it.
Or perhaps we could have a lifeboat situation. A planetary native captures a starship Captain and orders him (on threat of death) to use his advanced means to destroy an enemy. In that scenario it's conceivable a Starfleet officer might sacrifice themselves. This, of course, is an extreme situation, but perhaps we get a glimpse of it in "Bread And Circuses." The Proconsul demanded Kirk surrender his entire crew or he and his party would be put to death. Kirk refuses and it really does look like they're done for until Scotty's intervention grants them a chance for escape.