Regarding the hostage issue, I don’t see how feeding a hostage makes him a guest, since he is clearly being held against his will, even if he is accorded freedom of the castle or his parole is accepted not to return home. You feed all sorts of other prisoners as well, such as criminals and war captives. It does not give them guest-right, nor impose any duty to their hosts, or they’d have more of a stink about Jaime participating in his escape attempt, tho from Cat’s words, parole was implied?

There was a stink about Jaime participating in his escape attempt! 

“Jaime got hold of a sword, slew Poul Pemford and Ser Desmond’s squire Myles, and wounded Delp so badly that Maester Vyman fears he’ll soon die as well. It was a bloody mess. At the sound of steel, some of the other red cloaks rushed to join him, barehand or no. I hanged those beside the four who freed him, and threw the rest in the dungeons. Jaime too. We’ll have no more escapes from that one. He’s down in the dark this time, chained hand and foot and bolted to the wall." 

And as I suggested in the previous post, there is a need to establish a duty of care so that the custom can function – to prevent hostages from attempting to escape, to ensure that hostages will not be murdered out of hand (although there are some violations of this, see Qhored the Cruel) so that people will be willing to hand them over and stop fighting. Indeed, the custom of parole is a good analogy – if there’s no duty imposed on both sides, no one’s willing to surrender and lots of needless death takes place.

However, there are limits – hostages clearly can be executed if the person who handed them over violates the terms of conduct that they had agreed to. For example, see Gerold the Great, who hanged hostages each time the Ironborn raided the Westerlnads. Even in this case, however, in order for the custom to function, there needs to be a common understanding of the causality of their deaths, so there’s still an incentive to disincentivize unnecessary deaths.