If you cast a spell using Malcolm, Alluring Scoundrel's last ability, you do so as part of the resolution of the ability. You can't wait to cast the spell later in the turn. Timing permissions based on the card's type are ignored.

You may not play land cards discarded with Malcolm, Alluring Scoundrel's last ability.

If you cast a card "without paying its mana cost", you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.

If the spell has {X} in its mana cost, you must choose 0 as the value of X.

If Malcolm, Alluring Scoundrel isn't on the battlefield as its triggered ability resolves, you won't put a chorus counter on it, but you'll still draw a card and discard a card. You may still cast the discarded card without paying its mana cost if Malcolm had four or more chorus counters on it when it was last on the battlefield.