Why didn't Jin include Mantis in his build to replicate the effects of 'Fujin'?

The whole thing that makes Fujin such a deadly weapon is the fact that it can phase through solids and then attack the opponents from very unexpected angles.

Which other attacker weapon could do that? Scorpions! The mole claw move could do it, Kazama being a good example of this. Now moleclaw with a regular scorpion would only work in extreme melee attacks, but this could change if a scorpion from the main and sub slot each where combined to get a Mantis which allows for performing moleclaw extended range without sacrificing trigger durability.

This means that using mole claw along with Mantis could somewhat replicate the effects of Fujin( except the range, of course)?

The only weakness is the inability to equip a defensive trigger, but with Jins clairvoyance, it covers that base.