Why does Frank come back to face Death in OUATIW?
Because he prefers not to die of curiosity about whom Harmonica really is.
... it has nothing to do with the other examples, where characters are brought to act against common sense to serve the necessities of the plot.
Well, it comes pretty darn close now, doesn't it? The whole "man to man thing" is, if anything, a necessity of the plot.
...the triello, though being effective, is just a gimmick, done for the hell of doing something that was never done before.
That's absolutely true but what would have been your "conclusion of the plot"-ending to GBU alternatively? Angel Eyes blasting Blondie and Tuco, then searching for the gold? Shrewd as he is?