Ride the Shoopuf's last ability doesn't have a duration. Once it resolves, it will remain in effect until the game ends, Ride the Shoopuf's leaves the battlefield, or some subsequent effect changes its characteristics, whichever comes first.

If Ride the Shoopuf becomes a creature but you haven't controlled it continuously since your most recent turn began, you won't be able to attack with it that turn.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

Whenever a land you control enters, each landfall ability of permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve. As a result, you can have those abilities resolve in the order of your choosing.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.