Hmm, seems like nobody here really knows how a monte carlo sim works, anyway, yes. you can trust simcraft, you just need the correct TCI input. In the case of Scabbard of Kyanos, simply replace the 'use_item', under the actions section, if you're using the base Ret APL.
Code:
actions+=/use_item, whatever
with:
Code:
actions+=/use_item,name=scabbard_of_kyanos,if=buff.avenging_wrath.up
This will allow the sim to use scabbard only when avenging wrath is active, i assure you, you'll notice a massive increase.