If the targeted creature has become an illegal target by the time Intimidation Bolt resolves, Intimidation Bolt doesn't resolve. Creatures will still be able to attack that turn.

If the targeted creature survives the damage, it can attack this turn, but no other creatures can. If the targeted creature is destroyed, no creatures can attack this turn, including creatures that enter later on.

For the second part of Intimidation Bolt's effect to do anything, it must be cast before attackers are declared.