how would world war 2 turn out if John Nance Garner was leading the helm?
Badly, because Garner was a staunched isolationist, so he'd take some persuading to enter the war in the first place. If he does, reluctantly declare war because of Japan forcing his hand, he'll probably have trouble convincing the public to go along with a war he wasn't keen on himself-and as stated above, he really doesn't have anything like FDR's Carrisma.
That said, it's dificult to get him in office during World War II in the first place-the only way to do it is if FDR dies sometime before the 1940 election. If FDR stands aside in 1940, it won't be in favour of Garner-he'll do all he can to make sure Cactus Jack doesn't get the nomination.