I did NOT learn what hideous musical style that King of the Forest song is supposed to be. Seriously, it's just embarrassing.
It sounded a lot like Gilbert and Sullivan, or more like a mockery of G&S.
And Gilbert and Sullivan is itself a mockery of grand opera. So I guess it's sub- sub-operatic in style. I sure could have done without it, especially as the Cowardly Lion had already had his "If I Only Had..." song just the same as the Scarecrow and the Tin Man. I don't see why he rated an extra song to himself.
As for the tin question, I think "tinned" used to refer to the INSIDE of a container. I know that copper jelly molds were coated with tin on the inside, where the food was poured. So "tinned" cans were coated with tin on the inside, to keep the metal from tainting the contents. The outside could easily rust, but the inside would still be good.