How much damage does a creature take if critically failing a Reflex Save with Improved Evasion?

Improved Evasion (available on at least Rogues and Swashbucklers) provides

You elude danger to a degree that few can match. Your proficiency rank for Reflex saves increases to legendary. When you roll a critical failure on a Reflex save, you get a failure instead. When you roll a failure on a Reflex save against a damaging effect, you take half damage.

How much damage does such a creature take on a critical failure? Full or half?

Does Enervation bypass Evasion only when Enervation is upcast?

Enervation, a 5th level spell from Xanathar’s Guide to Everything, has the following text:

The target makes a Dexterity saving throw. On a successful save, the target takes 2d8 necrotic damage, and the spell ends. On a failed save, the target takes 4d8 necrotic damage, and until the spell ends, you can use your action on each of your turns to deal 4d8 necrotic damage to the target.

And

When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.

First, it seems clear that all damage is increased by 1d8. For instance, Witch Bolt only increases the initial damage by 1d12, and with Ice Knife, the cold damage is boosted, but not the piercing damage.

Now, suppose I cast Enervation at 9th level and the target succeeds its DEX save. As written, the target takes 6d8 damage. Very unusual. Normally, I’d roll ALL THE DICE (8d8 in this case), and then halve the result for a successful save.

But speaking of halving the results, consider the Evasion feature for the Monk and Rogue classes:

When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

So: when does Evasion work against Enervation? Obviously not at 6th level or higher, but will Evasion activate when Enervation is cast at 5th level? Is 2d8 considered half of 4d8?

Evasion and Vulnerability to Energy precedence?

A creature has Evasion:

‘If she makes a successful Reflex saving throw against an attack that normally deals half damage on a successful save, she instead takes no damage.’

And also has Vulnerability to Energy:

‘Some creatures have vulnerability to a certain kind of energy effect (typically either cold or fire). Such a creature takes half again as much (+50%) damage as normal from the effect, regardless of whether a saving throw is allowed, or if the save is a success or failure.’

Which rule takes precedence if a creature is attacked by an effect of that energy type and a successful Reflex saving throw is made against this energy attack that normally deals half damage on a successful save?

Is there any way to gain access to Evasion without taking multiple levels of a class?

So, I have a character with a really, really, really bonkers Reflex save. It wasn’t really the design plan, but a combination of class features, ancillary benefits from traits, and stats meant that I ended up with one.

Given that, Evasion would be a really great feature to have – better still, the Twist Away feat, which requires Evasion, would be incredible for this particular character, and thematically fitting.

Problem is, dipping 2 levels of Rogue (or another class) is a larger setback than I’d like.

So, is there any way to get Evasion through a feat or other means? I’m particularly interested in permanent methods, in order to qualify for other feats. One level dips are worth mentioning, but not ideal, and if it helps any, the character is Mythic, on the Trickster path, and can expect to get to Tier 10 ‘someday’ (but not anytime soon).

I’m interested in solutions that might apply for any race/class/level combination, as it seems a handy thing to pick up in a variety of circumstances – especially since Twist Away exists. That said, my particular circumstance is a Half Elf, Swash 1/Investigator 7 (the ACG versions).

Does Evasion allow half damage even when unconscious?

Evasion is a class feature gained by Rogues and Monks at level 7:

At 7th level, your instinctive agility lets you dodge out of the way of certain area effects, such as a blue dragon’s lightning breath or a Fireball spell. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

Does this feature allow a character to “dodge” area effects, taking half damage, even if they are unconscious?

How does Shield Master interact with a Rogue’s Uncanny Dodge and Evasion?

Shield Master: a) add shield’s AC to Dexterity saving throw; b) for Dexterity saving throws to do with effect you receive either no damage on a save; half damage* on a fail. (PHB p.170)

Uncanny Dodge: use Reaction to only get half damage* done by an attack. (PHB p.96)

Evasion: for area of effect damage requiring a Dexterity saving throw, you receive either no damage on a save; half damage* on a fail. (PHB p. 96)

Does a rogue’s Evasion work against spells that don’t target an area?

At 7th level, rogues gain the Evasion feature:

Beginning at 7th level, you can nimbly dodge out of the way of certain area effects, such as an ancient red dragon’s fiery breath or an ice storm spell. When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.

The Immolation spell certainly allows a Dexterity save for half damage, but it is not an “area effect”:

Flames wreathe one creature you can see within range. The target must make a Dexterity saving throw. It takes 8d6 fire damage on a failed save, or half as much damage on a successful one. […]

Given that Evasion mentions dodging “certain area effects”, does that prevent it from working spells that do not target an area, such as immolation? Or does evasion apply to all Dexterity saves for half damage?

(Other example spells include enervation and flaming sphere.)

Does a Rogue’s Evasion work for spells?

The Rogue class at level 2 gain the ability Evasion:

At 2nd level and higher, a rogue can avoid even magical and unusual attacks with great agility. If she makes a successful Reflex saving throw against an attack that normally deals half damage on a successful save, she instead takes no damage. Evasion can be used only if the rogue is wearing light armor or no armor. A helpless rogue does not gain the benefit of evasion.

Does this also apply to spells that have a Reflex save for half damage, such as Fireball?