Each time the landfall ability resolves in a turn after the second time, you'll just add mana.

If you reveal your entire library and don't reveal an Elf or Elemental card, you'll put all revealed cards back in a random order. This uses the same physical action as shuffling, although it isn't technically a "shuffle."

Nissa's landfall ability isn't a mana ability, even though it causes you to add mana. It uses the stack and can be responded to.

Whenever a land you control enters, each landfall ability of the 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 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.

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