1&2 The Darkin are Ascended sealed within weapons, so they are around that power level. I'd assume in Aatrox's ult, his power level returns to being one of the highest among the Aspects and Ascended. The current assumption is that celestial magic is effective on the undead and that Aatrox has slain demigods before.
3&5 Aatrox returns to being trapped within the sword, notice the host body disappears but not the sword. The same happens to Rhaast (he returns into the scythe), its probably that Kayn and Rhaast can't have differing death animations though, hence Rhaast's host body doesn't fade away.
4 Yes, however not single handedly. He was once an Ascended and possibly has some insight to the workings of Targon. If he manages to steal the power of an Aspect, imagine what could happen. If he manages to topple Targon, destroying Runeterra could become and achievable feat for him. However keep in mind his true goal is his death/ freedom, ideally someone would find a way to kill him long before then.