Renee will have no issues killing the dragon, and this dude won't be able to do his mission. Poor dude, but it's his fault for choosing to serve someone who will do anything to achieve their goals, no matter how scummy.
Well, if Renee is the one who bring back the item because of the quest, then technically Idin's objective is fulfilled (prevent Isir from getting it)
The alternative where Isir get the credit would also means ISIR get Sedin and I highly doubt Renee is ok with that route.
I'm so tired of seeing 4d chess manipulator/schemers having the most brain dead plans. The princess is making an assumption of they'll be tired but they know it's 2 sword masters and a sword masters master group. If you as an author are not smart enough, then don't make these archtypes as you just look incredibly stupid and the villain/antagonist look retarded.
Except nobody is aware of Renee's true power level (even Sedin who's aware she can use magic + is sword master doesn't know about her constellations)
It's not 'brain dead plans', it's working under incomplete information that leads into false assumption (that the 3 'swordmasters' are about equal in power, given Isir and Sedin as existing 2)
On the chapter:
Random thought - what if the plot twist is Sedin is her pet dog reincarnated? Man's giving some Nine vice here.