Critical Strikes and Miss chance

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Malky wrote:

This is a rather clumsy monster so it has no chance to evade nor block.

Now our valiant hero is still quite fresh in his adventures and has a 90% chance to hit his foe, 5% chance of it being a devastating critical hit.

Leaving aside that mosnters can't take Unwavering stance and thus always have the minimum 5% chance to evade, this situation is impossible. If the monster can't evade, you by definition have 100% chance to hit. If you have only 90% chance to hit, then the monster by definition has 10% chance to evade.

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Malky wrote:

Option 1:
The numbers 1-90 is registered as a hit, 91-100 as a miss, incase of a hit, a second roll is made with the numbers 1-5 resulting in a critical strike.

This is correct. Chance to hit and chance to crit are completely unrelated mechanics.

Actually, "completely unrelated" is a lie.
Chance to crit is rolled completely separately from chance to hit, but if you roll a crit, you then have to check chance to hit again to 'confirm' the crit - if this second chance-to-hit check fails, the hit still hits, but doesn't crit, if it hits, you get the crit.
This is done to help protect evasion users from getting crit no the one hit they fail to evade.