The new theater season has been competing with the obituary pages. Four Broadway-bound shows, Banderol, A Matter of Position, There Must Be a Pony and La Belle were entombed en route. Step on a Crack limped into New York last week minus two successive leading ladies (Rita Hayworth, Nancy Kelly), and with an unknown understudy played a one-night stand. Come on Strong, Garson Kanin's nonplay about how to succeed by really sleeping around, posted a closing notice, then rescinded it, and is apparently hanging on by comely Carroll Baker's sliding shoulder straps. Manhattan's Seventh Avenue has been pilfered, as it is a couple of times a season, for a spotty cloak-and-suit comedy called Seidman and Son that is full of decent sentiments and indecent sentimentality. A play it isn't, but thanks to Sam Levene, that endearingly amusing one-man encyclopedia of Jewish gesticulation, box office it may be.