Al Smith runs for third party in 1932, splitting the Democratic vote. Hoover wins a second term thanks to the Democratic split, similar to Wilson in 1912. He keeps prohibition as it was Republican policy at the time
(1) There's no way that Smith would do this. In 1932, he still considered himself a loyal Democrat despite his bitterness toward FDR.
(2) If he did, there is no way he could enable Hoover to defeat FDR. Indeed, because his disagreements with FDR were from the Right (insofar as there *were* any ideological disagreements--they certainly were not as sharp as in 1936) he might take votes from Hoover as well as from FDR. (Yet he could not get financing from conservative Dempcrats like Raskob--as much as they distrusted FDR, they thought they had saddled him with a sufficently conservative plarform and were supporting him that year.) But he would not get many votes anyway--he was no TR so far as national popularity was concerned, and everyone who disliked Hoover would know the only way to get rid of Hoover would be to vote for FDR. Moreover, he could not make any TR-like claim that he had won the primaries--he won them only in MA and NJ and got far fewer votes overall than FDR did.
https://en.wikipedia.org/wiki/1932_Democratic_Party_presidential_primaries A Smith candidacy would just look like pure personal bitterness, "sour grapes"--no distinctive platform like that of the 1912 Progressives, no sense that "we stand at Armageddon and we battle for the Lord."
The very best he could do might be to deprive FDR of a close northeastern Catholic state like MA--but FDR could lose the entire Northeast (which he wouldn't) and still win.
(3) Even the Republican platform in 1932 came out for resubmission of Prohibition to the states. The 21st Amendment was in fact passed by the lame duck 72nd Congress while Hoover was still president. Ratification was not a matter of Demcorats versus Republicans-- of the twelve states that did not ratify by Decemeber 1933, six were Democratic-controlled southern states (SC, NC, GA, LA, MS, and OK), and in fact the only states to reject the Amendment outright were SC and NC.