What happens if someone casts Dissonant Whisper on my halfling?

Specifically, what do I get from my Brave halfling ability:

  • I roll 2d20 for the Advantage, and this "succeed or fail the save" is directly applied TO THE ENTIRE SPELL.

or

  • I roll 2d20 but I roll them "in order" (we use 2 different colors to tell which is d20 #1 and d20 #2), and the result is applied TO THE EFFECTS OF THE SPELL, CHECKING IF THE ADVANTAGE DICE APPLIES OR NOT TO EACH SEPARATE EFFECT. Meaning, the 2nd d20 is used for Advantage only against the specific "vs Frightened Condition" part of the spell , while only the 1st d20 aka "I do not have Advantage" is used against the damage part of the spell.

That DM does that kind of logic for everything else too, and he just loves to use "damage + several different effects" situations. Such as a DEX Save for half damage vs 8d6 damage half fire and half bludgeoning, let’s say when escaping a burning room full of flaming falling wooden beams.

All players think it should be done the 1st way. Simple enough, right?

But the DM says that it "obviously" has to be done the 2nd way, while also insisting that this is not a house rule, but that this is the real "RAW" way to interpret how Advantage on Saves should work.

Please help un-confuse us!