[RPG] Is this the correct way to resolve a missed Ice Knife

The damage will remain 2d6 if the save is failed.

Where a spell effect calls for a saving throw, critical damage is not in play. Reviewing spells like lightning bolt, fireball, and thunderwave that have multiple dice for damage, nowhere does the spell effect describe a doubling of damage dice for critical hits.

Hit or miss, the shard then explodes. (p. 19 Elemental Evil Player's Companion)

What this indicates is that the explosion damage is not linked to the attack roll's success or failure, and thus cannot benefit from the auto hit or critical on a 20, nor the auto miss on a 1. (p. 194 PHB)

Ice knife is an unusual spell in that it has two separate effects. One effect is an attack roll (hit or miss, and thus eligible for a critical hit benefit), while the other effect is the standard save versus damage.

Compare to other elemental spells

As a point of comparison, see this last part of the spell description (p. 19 Elemental Evil Player's Companion):

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the cold damage increases by 1d6 for each slot level above 1st.

This is comparable to Lightning Bolt's similar "at higher levels" language. From PHB p. 255:

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.

That elemental spell effect is also influenced by a saving throw and has no "critical" hit feature.

Conjure Barrage is not a ranged weapon attack.

This does not work because "ranged weapon attack" in D&D 5e is a term of art--it refers to a specific piece of the rules, and not necessarily the underlying fiction. While the effect of conjure barrage may intuitively seem like a ranged weapon attack, it is not a "ranged weapon attack" as far as the rules are considered.

A "ranged weapon attack" is partially defined on PHB 194:

When you make an attack, your attack roll determines whether the attack hits or misses. To make an attack roll, roll a d20 and add the appropriate modifiers.

On the other hand, conjure barrage asks for a saving throw—it is the target that rolls the dice in that situation. Because it calls for a saving throw, and you don't make the attack, it doesn't work.

Compare this situation to a spell like fire bolt, which asks you to make a ranged spell attack. In this case, you're making an attack, so you roll the d20, but it's still not a "ranged weapon attack" because it's a "ranged spell attack".

Essentially, you need to fulfill all three words of the phrase. You must make an attack (you roll the attack roll), at range, using a weapon.

Note that you can't cast those spells in one turn.

The wording in your question is a bit ambiguous, but it's important to note that you can't cast both of those spells in one turn, because that would require a bonus action spell and a 3rd level spell. PHB 202 states, (emphasis added)

A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.