Short answer: yes.

Can you foretell as an instant?

Can you use forecast ability at instant speed? Yes. You can pay two mana and exile a card with foretell from your hand at instant speed – but only during your turn.

Do instants have summoning sickness?

On her end step you play Midnight Haunting. When it's your turn, since you've controlled the tokens since the beginning of your turn, they do not have summoning sickness.