The Ball mode of the Plasma Blaster causes splash damage. Thus the alien will not use it when standing near you.
Not relevant. None of the units involved in the depicted situation was wielding that weapon.
And I know about this; I choose the ball mode for plasma blaster reaction fire only if the unit isn't standing in smoke since I expect it to not fire in this case. I mean, it would still be better to fire, get damaged, kill and thus stay alive, instead of not fire, stay unwounded and get killed. The decision is wrong, but at least logical. If there's a chance to harm civilians, allies or self, don't fire, that's how I would implement it in the first place.
Did the alien move next to you before?
Yes. And IIRC it was without smoke this time.
Maybe it triggered the reaction fire when it was still out of range?
I don't understand. In this case it shouldn't trigger, but instead "save" the attack for when the range condition matches, should it ?