Magic Resistance and Destructive Wrath

Currently hit a debate within our campaign in regards to gaining advantage on a Saving Throw from Magic Resistance when our Tempest Cleric can use their Channel Divinity for Destructive Wrath.

I have already checked these two posts: How do I know if an ability is magical? and When does a Tempest cleric have to choose to use Destructive Wrath?.

If the Cleric uses his Wrath of the Storm ability to deal Thunder/Lightning damage the target has to make a Dex Save. As per the Sage Advice this is not magical and therefore isn’t made at advantage by a creature with Magic Resistance.

If the Destructive Wrath is used as a Channel Divinity it gains the wording "fuel magical effects" and would therefore become magical for the purpose of Magic Resistance, however the most upvoted post on Destructive Wrath puts the damage being rolled after the saving throw and thus applied after the saving throw is made.

My question is what would be the best way to apply the rules in this example?

Would you apply the Destructive Wrath after the saving throw has been made thus detrimenting the creature, or would you apply the Destructive Wrath before the saving throw thus (potentially) detrimenting the player?

Would a race with bludgeoning, piercing or slashing resistance be overpowered?

The various subtypes of dragonborn in D&D 5e have different elemental resistances (acid, cold, fire, lightning, poison). Tieflings similarly have fire resistance, and I would estimate that a variant tiefling with, say, cold or poison resistance instead would be equally balanced.

However, suppose I want to give a race resistance to one of the physical types: bludgeoning, piercing, or slashing. For example, a dragonborn subtype with slashing as its one resistance, or a tiefling with piercing resistance.

Would this be overpowered?

Does Spell Resistance scale when advancing a monster?

I just advanved a Canoloth (MM 3, p. 200) as I wanted to use it against a lvl 14 party. It is originally CR 5 and I wanted to raise it to CR 8. So I added 4 HD (CR+2) and the elite array (CR+1). This raises a lot of stats, especially saves, but against a lvl 14 party Spell Resistance 18 is miserable. Effectively the Canoloth lost half its Spell Resistance compared to basic Canoloth matched against a lvl 11 party. So would you judge that raising SR to 21 would raise the advanced Canoloths CR above 8 or should this just be part of the advancement to CR 8?

How can I gain resistance to piercing, bludgeoning, and slashing damage while in heavy armor?

I am theory-crafting a character that takes very little damage. I want to use the Heavy Armor Master feat to help with damage reduction from non-magical attacks, as well as the Battlemaster maneuver parry, and the Goliath feature stone’s endurance. How can I gain resistance to piercing, bludgeoning, and slashing while also using the Heavy Armor Master feat? I am most interested in answers that don’t depend on external sources, such as other characters and magic items, but I’d like to hear answers including those as well. Multi-classing is an option.

Do Goliaths get cold resistance?

There seems to be a discrepancy between sources for the Goliath’s ‘Mountain Born’ racial trait:

Volo’s Guide to Monsters says:

Mountain Born. You’re acclimated to high altitude, including elevations above 20,000 feet. You’re also naturally adapted to cold climates, as described in chapter 5 of the Dungeon Master’s Guide.

(Btw there is nothing in the Volo’s Errata to correct any of that)

D&D Beyond says:

Mountain Born. You have resistance to cold damage. You’re also acclimated to high altitude, including elevations above 20,000 feet.

Which is it?

Do creatures with Legendary Resistance know what is hitting them in order to choose to use the resistance or not?

Say I cast a Fireball at a white dragon, it definitively knows that the spell cast was a Fireball, with all those flames around…

However, some spells are more subtle, such as Banishment. Maybe the characters want to have about 1 minute to build something that will kill the dragon once it returns… but then it could take casting the Banishment spell (or other spells) many times to successfully sending the dragon on a demiplane for a while.

Yet, if the Dragon (or other target) cannot really know what the spell is, then it could accept a failure thinking it may need its resistance later when it is in a worst situation than at this time…

I have not been able to find something in RAW that says the target knows of the exact spell effects when it has the Legendary Resistance skill. Is there?

Scrying on a creature with legendary resistance

My players recently decided that they would like to cast the Scry spell on a creature that has the Legendary Resistance trait.

Does this trait function outside of combat?

My major concern is that the creature wouldn’t necessarily know it was being scryed on in order to use the resistance to avoid it. In combat, a creature could see a (for example) fireball coming and decide to avoid it, but with no indicator at all that a spell was targeting the creature, it is less clear to me.