What happens when taking damage during Wild Shape and trying to use the Absorb Elements spell?

Question

The Wild Shape feature includes the following:

[...] if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. [...]

Meanwhile, the absorb elements spell reduces damage taken, even the triggering damage source. And from the spell description we can also see the following:

1 Reaction [...] which you take when you take acid, cold, fire, lightning, or thunder damage [...]

So what happens if a low level Wild Shaped Druid has 10 HP left in their Wild Shape form and takes 30 fire damage?

They cannot cast the spell at first because they are in Wild Shape, but then, after 10 damage, they revert to their normal form. Can they then cast the spell?

If they can cast the spell, does it only impact the remaining 20 damage or does it have some sort of retroactive effect on the damage they already took?

If it does have a retroactive effect, what happens if they initially only took 16 damage?

How does the absorb elements spell interact with Wild Shape?

Answer 1

In this scenario, the character is taking damage, becomes able to use spell reactions mid-damage, and the absorb elements spell reacts to taking damage, so I see no reason to stop the character from using this spell as you describe.

The Sage Advice Compendium helpfully answers how to deal with resistance that suddenly appears when a transformation ends. While this answer is directly dealing with always-on resistance (such as a Dragonborn's innate resistance or resistance gained from a magic item that was previously merged with the transformed state and has now re-activated as the transformation ended), it pretty clearly answers your questions about whether the resistance is retroactive:

If a creature under the effects of polymorph takes enough fire damage to revert to its true form and that form has fire resistance, does the true form take the full remaining damage or only half due to resistance?

When the creature reverts to its true form, any leftover damage is subject to that form’s damage resistances, if any.

So if the wildshaped form had 10 HP and then took 30 fire damage, the first 10 points break the transformation, and then the remaining 20 fire damage that carries over is halved by resistance to 10 actual damage against the character's HP.