Companions/Sidekicks: which should not be single-classed?

You should not shoot at your folks with Blinding Smoke since it's friendly fire (Avenging Storm will also hit them then!). But: it's a cone that spreads out behind the target you crit. So your party members usually are save if the stand between you and the enemy. A Streetfighter with Deltro's Helm (who doesn't use blunderbusses himself) actually might want to get hit by it though: it gives him flanked and a shocking lash. Just watch his health if you fire at him often.

Items/buffs: everything that gives you bonuses for ranged combat and shock dmg and PLs and PEN (Ring of the Marksman, Heart of the Storm, Deltro's Cage). Nature's Mark is an excellent spell for a mortar user (lowers deflection and reflex and both get targeted by mortars).  If you have a Wizard I would use Miasma a lot to lower Reflex by lowering PER which also stacks with DEX afflictions and so on. Also the synergy between Pull of Eora + Combusting Wounds with a mortar firing/Avenging Storm casting Tekehu is nice. Bringing a Priest with Devotions and a Paladin with Zealous Focus helps, too.

But the basic concept is just: having fun with mortars + Avenging Storm.  Watershaper's Focus also works nice, but the recovery malus for Blast is too hefty for my taste.