First of all, let's keep all battle mechanics questions in one topic.
Now, let me correct some of your misunderstandings.
1) Damage increase by 1 % (minimum and maximum) for each 4 strength points or 0.1 damage (minimum and maximum) for each 4 strength points.
You gain 1% bonus to your base damage for every 4 strength points, not 0.1 damage for each 4 points. For example, if I have 100 strength, it will give me +20% to my damage, not 2,5 damage. +20% can be 2,5 damage if your weapon's damage is 10, but it can also be 80 if your weapon's damage is 400.
2)Determines the hero's chance for successful attack (enemy's agility is taken into account as well). With both your dexterity and enemy's agility equal, the chance for successful hit is 75%.
This doesn't mean that the maximum chance to hit is 75%. It meens that it's 75% when those stats are equal. If for example you have 150 dexterity and your enemy has 40 agility, you will hit 100% of the time.
3) I know there is a maximum hit % of 75% but are there a minimum hit % or is it able that one can get 0% hitting chance?
The minimum chance of successful hit is 35%. For example, if your enemy has 50 dexterity, even if you have 200 agility, or even 500, he will still hit you once every three attacks.
4) Damage = Maximum damage of weapon(including all bonuses) - damage reduced by total armor . If that damage is higher then minimum damage of weapon, then that damage is applied in successful hit. If that damage is lower then minimum damage of weapon, then minimum damage of weapon is applied. Critical hit multiplies maximum damage with 3. Then armor is applied. And the rest is the same as in normal hit."
This is correct, however there is also one addition. There are some body armors that give "Damage reduced" bonus. This reduction is applied after the damage calculations. This means that if your armor brings enemy damage to their minimum, the damage reduction bonus will bring it even lower.
5) My hero have a totale armor of 5054 (421 damage reduse points) with a plate armor and haevy shield. If I put a chainmail armor on it will lower my armor 368 (30 hit points) but will boost my agility hit 20% from 51 to 61 what give more 10% chance to avade a shot. Most enemy weapons damege is lower that my armor reduse points (421). To me it means that I am over armoring my hero at the cost of agility. The armor help with the critical hit but if I can evade the git from the first place it is better for me..
It may still not be enough though. Check your enemies' average dexterity. By your armor values I'd say you're around tier 3 right now. 50 or 60, or even 70 agility will probably not make enough difference to bring your enemies' chance to hit lower than 100%.
What Crassus told you about strength and block is also true.
You should write your example battle again, with these new info taken into consideration so we can see if you're accurate enough or if further explanation is required.
P.S.: Choosing your equipment is not an easy task and in most of the cases, there's no "right" or "wrong" choice. Make a plan in your mind and see if it works.