Criticals like Weapons Faillure have cumulative efects? Like, if you have two of these, you roll less TWO dice instead of only one less?

Correct. Or if you have two engine failures, your blue 2 hard turns become red.

Yep. Each effect is added on its own, just as each "action" on them to repair them is treated as their own action (and not the same action on multiple cards) and each one can be triggered by the same event (multiple Stunned Pilot crits means multiple damage for overlap; multiple Disabled Power Regulator means gaining multiple ion tokens as well as all of them being repaired after an ion maneuver).