1. do all units have some kind of Hit Points? how much are their hit points?
Yes, unless stated otherwise all units have 1. The units with 2 hp are Generals Bodyguards and Hasashim i believe in vanila.
2. what does attack skills really mean? I mean for example what does attack point = 1 mean? does it mean that every time the unit punch an enemy unit it will deliver some kind of damage for 1 point?
It's actually attack factor, it's the value used when calculating the success of a hit. It's not a direct 1 point damage, this is not an rpg with weapons dealing out 9 damage to a guy with 20hp.

3. what are the effects of attacking animations? are the effects only depended on how fast the animation being executed?
Basically yes.

4. what about defense skills? (like question number 2)
Defense skill is a value sued in melee which adds to a units total defense, the higher it is the more likely a soldier in a unit will survive a hit made on them.

5. just like question number 3.
There aren't really any defense animations.

6. how do the game decide a unit executing attacking or defending animation during a fight?
Generaly it's the units in the stand ready stance being hit buy a guy doing an animation, and then vice versa, msot likely done at a certain percentage chance.

7. what if unit A that have attack skills = 5 attacking unit B that have defense skills = 5 (assume that unit B never counter attack)? will unit B killed in 1 punch (now assume that unit B doesn't have any armor or shield)? or is it just make unit B losing some hit points or something?
Like i said above it's not like an rpg, it's not like the stats in an rpg.

8. what do the differences between armor and shield in the game? assume that the attacker doesn't have armor piercing weapon and the defender is in defensive stance/animation, will 5 armor points be as good as 5 shield points?
If attacked from the front in melee yes. If attacked from the left or right side only half the shield value is counted and from the rear none of the shield value is counted.

9. how is the ranks between these from best to worst : a unit that has defense skill = 10 but only 3 armor and 3 shield, a unit that has 8 defense skills but 4 armor and 4 shield, a unit that has 6 defense skills but 3 armor and 7 shield, a unit that has 6 defense skills but 7 armor and 3 shield - all those assuming that the attacker doesn't have armor piercing weapon.
If from the front as the total defense is always the same there would be no difference. From the side and rear those with lower shield values would fare better.