How does additional damage work with the Hand Mortar's explosive property?

Question

So I am building a gunslinger, and see a possible very high risk, but very high reward tactic for large groups.

Say I have a hand mortar, and fire it into a creature surrounded by at least 2 others. The hand mortar has the explosive property

Explosive. Upon a hit, everything within 5 ft of the target must make a Dexterity saving throw (DC equal to 8 + your proficiency bonus + your Dexterity modifier) or suffer 1d8 fire damage. If the weapon misses, the ammunition fails to detonate, or bounces away harmlessly before doing so.

Now say I choose to spend 2 grit points on the Violent Shot trick shot.

When you make a firearm attack against a creature, you can expend one or more grit points to enhance the volatility of the attack. For each grit point expended, the attack gains a +2 to the firearm’s misfire score. If the attack hits, you can roll one additional weapon damage die per grit point spent when determining the damage.

Does this mean that if I hit and do not misfire (roll above a 7, and above the main target's AC) I deal 6d8 + DEX fire damage to the main target AND a potential 3d8 fire damage to the creature around the target, or does the "splash damage" not count as weapon damage?

This is entirely based on the DnD beyond Gunslinger subclass definition.

Answer 1

Violent Shot grants a weapon die

Per your Violent Shot citation (emphasis mine),

When you make a firearm attack against a creature, you can expend one or more grit points to enhance the volatility of the attack. For each grit point expended, the attack gains a +2 to the firearm’s misfire score. If the attack hits, you can roll one additional weapon damage die per grit point spent when determining the damage.

Tackling these in reverse emphasis order, Violent Shot adds to your weapon damage. The Hand Mortar does 2d8 + Dex fire damage and has the Explosive property, which deals additional fire damage to nearby enemies. To minimize semantic arguments, note that the target hit by the weapon does not take this additional damage*. It follows that the Explosive property does not confer weapon damage and therefore does not get amplified by Violent Shot. This is supported by a similar answer on a related question here.

The other word I've bolded is "one", though that's reinforced by the non-plural "weapon damage die". Your mortar's weapon damage dice are 2d8, but its die is a d8. As such, Violent Shot would add 1d8 per grit point spent. With two points, your total is 4d8 + Dex to your target, plus 1d8 to targets within 5 feet

*See this related question and its accepted answer featuring a ruling by Jeremy Crawford, the Lead Rules Designer for this edition