[X] Ask to have Steffon join your Progress first so you have a chance to get to know him better.
If we establish him as someone who simply attached themselves to the progress and proved competent rather than someone we recommended entirely on the recommendation of another, that gives Jeyne less ammunition for suspicion or protest. This meeting is private after all.
[X] [Defiant] Demand a blade.
Let him die by a woman's sword, his head and wagging tongue separated from his body and the badge of privilege he hangs between his legs.
Arnold Arryn even in canon seems like a really stupid dude. Completely fumbling two rebellions, and like he was a first cousin, I think even most of Westeros would be like "Nah, Jeyne comes first she was her lord father's only surviving child."
Joffery, in all of canon, was only this stupid once, when he took neds head, and someone had clearly put him up to it. To surpass Joffery in terms of failed genetics and nurture is a feat I thought never to see in westeros, but Arnold Arryn manages and then does a victory lap for good measure.
Edit: though, that there are idiots nodding along while the king has no male heir yet born in front of his eldest child is worrying indicative that idiots are, somehow, even more common in this century than another. I guess if Roberts rebellion accomplished one good thing it was kill off some of the morons in the realm.