>>2136793
Mostly just more, the way stat gain works in xcom is that it's in XP brackets, just refer to the table. TL:DR XP gain caps at 11 hits maximum.
Primary stats are bravery, reactions, firing, throwing, melee accuracy and psi skill.
Secondary stats are TU, health, stamina and strength.
Primary stats only increase if you use weapon of the skill that trains it, which is mostly self explainatory guns increase shooting, throwables and bows throwing, etc. BUT there are exceptions, just off the top of my head in Piratez ninja sickle will always train throwing even if melee is used and cobra staff will train psi skill even with melee attacks. Always check for the experience mode if in doubt.
>>2136801
As long you hit. Reactions are an exception to this role, you only have to react hitting is not needed.
>>2136860
>only one of the pellets count for ACC training. Is it true?
Yes.