Please explain how what I described is contradicted by your happily linked damage formula.
Both critical chance and attack power are affected by diminishing returns when there is overkill involved, however the critical chance ultimately leads to a flat critical damage bonus. The added benefit for increases of critical chance is greatly reduced because the damage bonus is set. The benefit of going from 1% to 2% is 2x, but the increase from 2 to 3 is 1.5x, 3 to 4 is 1.33, etc. Eventually you'll have enough critical chance that the increased chance of critical adds very negligible benefit, while +attack power is a flat increase in damage (but as with crit damage less useful if you overkill).
Additionally Bonus damage is a specific term, listed in skill passives, is Ability Multiplier here. Don't use the term bonus damage when it is used in the ability description, as it is in a different part of the formula, misleading the player. It is multiplicative with the total damage result, not additive like attack power is.
Your final damage is not limited to attack power alone.
You're misinterpreting relative increases in damage from attack power addition only. Attack multiplies with Critical, Flank, Guard, and Barrier Damage bonuses, and that result is subtracted by armor before being multipled by "bonus damage" / AbilityMultiplier. This multiplication affects final relative power increase results.
Not all of these numbers are straight additive, and everyone here is basing interpretations on flawed assumptions, no testing methodology, and no posted tested results.