Quoting relevant bits for Fiery Bolt:
Target: One creature
Hit: 3d6 + Constitution modifier fire damage, and creatures adjacent to the target take 1d6 + Constitution modifier fire damage.
The target is one creature, and that is the creature that's hit. It is the only creature for which damage should be maximized, and extra dice rolls from magic implement should be applied. Adjacent creatures are simply damaged, so that damage should not be maximized. Technically, the adjacent creatures are not even attacked.
Similarly, when you hit a target with cleave, the adjacent enemy is neither attacked, nor hit, they are simply damaged. A fighter does not get to mark them, and does not get to roll critical damage dice from a magic weapon.
Furious smash is a weapon attack that hits the target, so it would benefit from extra magic weapon critical dice.
For Dire Radiance and Hellish Rebuke, the secondary damage is not generated from a hit. It is generated as a consequence of another trigger, and as such, the damage should not be maximized.
Target: One creature
Hit: 3d6 + Constitution modifier fire damage, and creatures adjacent to the target take 1d6 + Constitution modifier fire damage.
The target is one creature, and that is the creature that's hit. It is the only creature for which damage should be maximized, and extra dice rolls from magic implement should be applied. Adjacent creatures are simply damaged, so that damage should not be maximized. Technically, the adjacent creatures are not even attacked.
Similarly, when you hit a target with cleave, the adjacent enemy is neither attacked, nor hit, they are simply damaged. A fighter does not get to mark them, and does not get to roll critical damage dice from a magic weapon.
Furious smash is a weapon attack that hits the target, so it would benefit from extra magic weapon critical dice.
For Dire Radiance and Hellish Rebuke, the secondary damage is not generated from a hit. It is generated as a consequence of another trigger, and as such, the damage should not be maximized.