You cast the instant or sorcery while Seifer Almasy's last ability is resolving and still on the stack. You can't wait to cast it later in the turn. Timing restrictions based on the card's type are ignored.

If a card in your graveyard has in its mana cost, X is 0 for the purpose of determining its mana value and you must choose 0 as the value of X when casting it without paying its mana cost.

A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable). For example, Seifer Almasy's first ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat. Similarly, creatures that enter the battlefield attacking later in combat won't be considered when determining whether or not a creature attacked alone.

If you cast a spell "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 spell.