If an instant or sorcery spell has an adamant ability, you perform the spell's instructions in order. You don't perform the adamant instruction before the spell resolves or before any other effects printed above it.

If an effect allows you to cast a spell without paying its mana cost, you can't choose to cast it and pay unless another rule or effect allows you to cast that spell for a cost. Similarly, you can't waive a cost reduction unless that effect says you may.

If you copy a spell that has an adamant ability, no mana was spent to cast the copy at all, so that ability won't apply.

Searing Barrage's adamant ability causes it to deal additional damage. It doesn't replace the damage dealt to the target creature.

If the target creature is an illegal target by the time Searing Barrage tries to resolve, the spell won't resolve. No player will be dealt damage, even if you spent three red mana.

Adamant effects check what mana was actually spent to cast a spell. If an effect allows you to spend mana “as though it were mana” of any color or type, that allows you to spend mana you couldn't otherwise spend, but it doesn't change what mana you spent to cast the spell.