I saw this play with my daughter, she's about Mel Brooks' biggest fan, and after the "Springtime for Hitler" big number that closes the act and brings down the house, I felt very guilty for applauding. I'm part German too, so maybe that had something to do with my guilt. I told the kid how I felt after the show and she said: Mom, the audience HAS to love that scene and go nuts, otherwise the rest of the play doesn't make sense!
Of course she was right, and I was then very impressed with how they make the audience really a part of the play, and get the needed effect, but so differently from the way it is done in the movie.
I think there's a point to be made about "The Producers" at least, that Hitler and the Nazis are always regarded as anthanema, that's why they think the show will fail. Unlike Hogan's Heros for example where "actual" Nazis are portrayed in a comical way. But I liked that show too, but that's cause Bob Crane (and Richard Dawson!) was soooo cute, what a shame the life Crane lived and the end he met.
I never saw the play, I'm referring to the 1968 movie with Zero Mostel and Gene wilder. Dick Shawn played the pothead who auditioned for Hitler. Piss-your-pants hilarious.