If you cast an instant or sorcery card this way, you do so while the ability of Jace’s Mindseeker is resolving. If you choose not to (or can’t), you won’t get a chance to cast one later.

“From among them” means the five cards put into the graveyard, not all cards in that graveyard.

When casting an instant or sorcery card this way, ignore timing restrictions based on the card’s type. Other timing restrictions, such as “Cast [this spell] only during combat,” must be followed.

If you cast a card “without paying its mana cost,” you can’t pay alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.

If a card has {X} in its mana cost, you must choose 0 as its value.

If the five cards are put into a public zone such as exile instead of a graveyard (perhaps due to the ability of Rest in Peace), you can cast one of those instant or sorcery cards from that zone.