[RPG] Are the Light Domain Cleric cantrips in addition to the 3 starting cantrips

The bonus proficiency is a domain feature. Domains are a class feature. The multiclassing rules state that you gain the class features when you get a new level in the new class. It then lists exceptions to this with channel divinity, extra attack, spell casting and unarmored defense. Domains are not listed, so it seems that the bonus proficiency is the more specific rule and you gain the domain proficienices.

The answer is sometimes. Green-Flame Blade says:

On a hit, the target suffers the attack's normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it.

You make an attack and resolve it normally, then it deals fire damage to an adjacent target. In this case you would only deal the extra damage to the second target because that is the only damage caused directly by the spell.

This changes however when you hit level 5

At 5th level, the melee attack deals an extra 1d8 fire damage to the target, and the fire damage to the second creature increases to 1d8 + your spell casting ability modifier.

At this point you would apply the extra damage to both targets, because the spell is actually causing the extra fire damage.