To reply to your simplest questions:

2 AGI= 1 evasion, 1 ranged accuracy. It can also be related to reducing enemy critical hit rate.

2 DEX= 1 accuracy. It is also related to your critical hit rate.

2 STR= 1 attack.

2 VIT= 1 defense


This is only a partial explanation, however. I need to do an event now so I can finish later if someone else hasn't yet.