Whether the target creature is still a legal target is not checked after Channel Harm resolves. Damage will still be prevented, even if Channel Harm can’t deal damage to that creature.

You can choose a creature you control as the target. If you do, the damage Channel Harm deals to that creature won’t be prevented.

If two players have each cast Channel Harm targeting a creature the other controls, and a source one player controls would deal damage to the second player or a permanent that player controls, one Channel Harm will prevent that damage and try to deal damage to the creature controlled by the first player. That player’s Channel Harm will then prevent that damage and try to deal damage to the creature controlled by the second player. This forms a loop that either player can break by choosing to not have Channel Harm try to deal damage. It doesn’t matter which player breaks the loop, as no damage will be dealt in either case.

Channel Harm’s only target is the creature it may deal damage to. You choose that target as you cast Channel Harm, not at the time it prevents damage.

If the target creature is an illegal target as Channel Harm tries to resolve, the entire spell doesn’t resolve. No damage will be prevented.

Channel Harm’s effect is not a redirection effect. If it prevents damage, you may have Channel Harm (not the original source) deal damage to the creature as part of that prevention effect. Channel Harm is the source of the new damage, so the characteristics of the original source (such as its color, or whether it had lifelink or deathtouch) don’t affect the new damage. The new damage is not combat damage, even if the prevented damage was.