Regarding 6-hit Pin Missile while burned: correct calculation is (2*6+3+(3-2)*1.5-3)+(-1-0)*1.75=11.75 damage (round to 12)
You do not calculate damage for one hit then multiply by 6. You take the base power per hit (in this case, 2), then multiply that by 6, then add in other modifiers like STAB and rank difference. The only things that get checked multiple times are crits (roll 6 times for crits in this case) and the occasional secondary effect like Garbodor's Stench Rock Blast.
Also, -1 attack means -1.75 to the final damage after weakness/resistance is calculated.