If you pay the dash cost to cast a creature spell, that card will be returned to its owner’s hand only if it’s still on the battlefield when its triggered ability resolves. If it dies or goes to another zone before then, it will stay where it is.

The ability that defines Kolaghan Forerunners’s power functions in all zones, not just the battlefield. If Kolaghan Forerunners is on the battlefield, its ability will count itself.

You don’t have to attack with the creature with dash unless another ability says you do.

If a creature enters the battlefield as a copy of or becomes a copy of a creature whose dash cost was paid, the copy won’t have haste and won’t be returned to its owner’s hand.

If you choose to pay the dash cost rather than the mana cost, you’re still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its dash cost only when you otherwise could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.