The First Sliver

The First Sliver's last ability only applies while it's on the battlefield. If The First Sliver's own cascade ability lets you cast another Sliver card, that Sliver won't have cascade.

If a spell has multiple instances of cascade (for example, if you cast a second The First Sliver), each instance of cascade triggers and resolves separately. The spell you cast due to the first cascade ability will go on the stack on top of the second cascade ability. That spell will resolve before you exile cards for the second cascade 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.