Can you use Dispel Magic on a Slowed Spellcaster?

Question

The scenario is as follows.

You've wisely cast slow on the enemy spell caster. They began on their turn to attempt to cast fireball, and rolled to see if they could complete it this turn.

They rolled an 11 or higher and thus will only complete it on their turn.

When your turn comes up, before the enemy caster has completed their spell, can you prevent it from completing by casting dispel magic?

Dispel magic states the following

Choose any creature, object, or magical effect within range. Any spell of 3rd level of lower on the target ends.

As the spell has not completed, is the process of casting fireball considered a spell, or is it not a valid target for dispel magic?

Answer 1

No, dispel magic cannot interrupt a spell

Dispel magic (PHB, pg. 234) says:

Choose one creature, object or magic effect within range. Any spell of 3rd level or lower on the target ends. ...

An enemy spellcaster currently casting a spell does not currently have that spell cast on them at that point in time, so Dispel Magic could not prevent the casting of that spell.

By contrast, counterspell (PHB, pg. 228) says:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. ...

Counterspell specifically calls out interrupting a spell. Dispel magic does not use this sort of wording, and is clearly intended to end spells that have already been cast rather than interrupting those being cast.

Answer 2

No, you can't interrupt a slowed casting of a spell with dispel magic

Fireball has not yet come into effect yet and thus there is no magical effect for dispel magic to target (besides slow). This stems from the duration of fireball, which is Instantaneous, and that duration is unchanged by virtue of slow (which only affects the casting time). For reference, a spell with an Instantaneous duration cannot be dispelled per p. 203 of the PHB:

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.

As a side note, I noted that it was interesting that by virtue of the extended casting time, you have a much bigger window within which to cast counterspell, which I'm visually imagining as some sort of deliberate effort to break the caster's concentration by flicking their nose.