Q:

Does the Flail Snail's "burst of destructive force" do no damage if it's triggered by a cantrip such as Eldritch Blast?

My party encountered a Flail Snail in our adventures, and the Warlock in the party had no real combat ability versus said snail other than the Eldritch Blast cantrip. The Warlock missed their attack, and as such the DM rolled for the Flail Snail's random effect. The way I am understanding this, only on a roll of 1 or 2 would the spell do anything other than miss.
The relevant passage, quoted from the flail snail's statblock (emphasis mine):

Antimagic Shell. The snail has advantage on saving throws against spells, and any creature making a spell attack against the snail has disadvantage on the attack roll. If the snail succeeds on its saving throw against a spell or a spell attack misses it, an additional effect might occur, as determined by rolling a d6:
1–2. If the spell affects an area or has multiple targets, it fails and has no effect. If the spell targets only the snail, it has no effect on the snail and is reflected back at the caster, using the spell slot level, spell save DC, attack bonus, and spellcasting ability of the caster.
3–4. No additional effect.
5–6. The snail’s shell converts some of the spell’s energy into a burst of destructive force. Each creature within 30 feet of the snail must make a DC 15 Constitution saving throw, taking 1d6 force damage per level of the spell on a failed save, or half as much damage on a successful one.

If I am understanding this correctly, that would mean an attack cantrip would only have a 33% chance of a negative effect on a miss, and on a roll of 5 or 6 do zero damage regardless since cantrips are treated as 0-level spells.
Is this interpretation correct?

A:

Cantrips are treated as "level 0" spells
From the description of cantrips in the basic rules:

A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.

So, for the purpose of the flail snail: if someone uses a spell attack cantrip on the flail snail and it misses, or someone uses a saving-throw cantrip on the flail snail and it makes the save, then it (or, well, the DM) rolls a d6. However, if it rolls a 5 or a 6, it actually doesn't do anything as written, because cantrips are level-0 spells - 1d6 times 0 equals 0.
Your interpretation is correct, for the rule as written. No minimum damage is specified.
(This may not be the intent, but you'd have to ask Jeremy Crawford - then maybe it'd be fixed in errata to do a minimum of 1d6 damage for cantrips.)

A:

On a 5–6, there is no force damage.
Cantrips have an effective spell level of 0, according to the rules for spellcasting:

A cantrip's spell level is 0.

So if you roll a 5–6 for the flail snail's effect which can deal 1d6 force damage per level of the spell, that calculation is equivalent to 1d6 times 0, which is 0 damage, regardless of the result of the save.
Since 5–6 would effectively do nothing and 3–4 explicitly does nothing, you are correct that only 1–2 could have an effect (which is 1 of 3 cases, or 33% of the cases). However, there is only a practical effect if the reflected attack actually hits the original caster, which means we would expect the probability of a negative effect at the table to be much less than 33%.

