Given the amount of political subterfuge in the book, I wonder if there couldn't be an argument made for the old Tisroc being less hostile toward the northern countries and more interested in keeping the peace.

I daresay the Old Tisroc might well be less hostile to the northern countries, having a good reason for it, when he didn't want to tangle with a dangerous White Witch. The present Tisroc seemed to have had much the same attitude to Narnia, itself right up until that fateful meeting with Aravis hiding behind the Tisroc's sofa in HHB. Until Rabadash took that trip to Cair Paravel to investigate the looks of the fair Susan and managed to spy out the land as well. The present Tisroc might have also known of the fall of the White Witch, since either he or his predecessor seems to have instigated spy expeditions into Archenland, which enabled, in turn, Lord Barr to steal away one of the King's twin sons. Even if no spying was officially recognised, there were also those people who lived by stealing away the likes of Hwin and Bree.

All that politicking around suggests to me that the bulk of the Calormenes were more interested in leading their own lives, that Tash-worship was more a ritual and a tradition than a useful faith, and an excuse for some of the Calormene leaders such as the Tisroc, himself, to behave badly. Emeth, the genuinely idealistic Calormene, surely wasn't the only one of his kind, but in his own Tarkaan class there seems to have been too many like Rishda Tarkaan who paid lip service to anything more than his own power and influence.

I thought the Calormenes in general were treated in quite a balanced fashion, in HHB, given that the society it depicts is definitely pre-television, and Internet, that Tashbaan could describe any city in the world where its civic leaders are more interested in enjoying themselves than in actually running the place, where bad practices such as widespread use of slavery have grown up. Even the Tarkaan Andradin was probably inspired by an actual historical character or two, the Barbarossa brothers. A couple of generations ago, there were so many people among those older people of the post war era who thought they wouldn't bother about girls' education because "girls only get married", and even in today's newspapers I read an article here about schools having to identify so-called "child brides" among students whose parents can't wait to marry off their daughters.

Yes, I like the idea of HHB being bookended like a story being read to a child. But there is something right at the end of Silver Chair, where Jill wears her Narnian outfit to a fancy dress party which enables HHB to be bookended anyway, in a social group, even though it isn't a bedtime story. The advantage of this idea is that it could be done in a context where we could briefly meet all the non-native Narnian characters we need for the remaining films. And it is so easy for Eustace and Jill in HHB, having joined such a group to mention right at the beginning that they had heard the story of HHB at Cair Paravel, and to ask Edmund about it. Eustace and Jill, having heard the story could then supply the missing information that Edmund and Lucy would not know, which was that Aravis and Cor did marry and go on to be the parents of Ram. They might even remember Prince Rilian happily whistling a tune about Corin Thunderfist's exploits with the Lapsed Bear. Especially if the tune unaccountably turns up on some radio station ot other.

The question is, how would you link the end of HHB to Magician's Nephew?
Personally I'm wondering if it would be better to link HHB after MN, because then we have the allusion to LWW at the end where we are told that Digory Kirk would be the Professor in that book whose wardrobe introduced Lucy to Narnia.

I love the idea of HHB being told as a framed story.
My thought was that both HHB and MN would be framed stories, both told at gatherings of the Seven Friends of Narnia. But HHB told at Cair Paravel would be great too.

HHB would have the same three actors for Peter, Edmund and Lucy in its story cast at their 'oldest' age and doing the story telling at the age they have reached in LB.

MN would have young Digory and young Polly in its story cast, while older Professor and Polly tell the story in the frame.

I have an idea for a framing device that could bookend the Horse and His boy film.
We open in a little boy's room that is dark. The boy is in bed supposed to be asleep but instead he is wide awake. Then the bedroom door opens a crack letting a little light in. A woman,his mother, pops her head through the door to check on her son. She sees that he's still awake; she admonishes him to go to sleep. The boy begs his mother for one more story. She says no and to got to sleep. He begs and begs and she gives in. She opens the door all the way and walks in with a lamp in her hand. She sets the lamp on the stand beside his bed. She sits down on the bed next to him and tells him this is the last one and to go to bed after this. He asks her for his favorite story.

Just as King Caspian appeared at the end of SC, it has occurred to me that Rilian, restored to Narnia, and now settled with a family, would also be a suitable candidate for your framing of HHB, especially as unlike the book SC the current film may not refer to him again after his reunion with Caspian, his father. In SC Prince Rilian might well have known quite a bit about this story, which was presented by minstrels at that Cair Paravel banquet at the beginning of SC. Rilian even whistled a tune connected with Corin Thunderfist once he was released from his captivity. So I'd imagine he would love telling his own son that story as well as or even instead of his own.

To me HHB is so action-and-dialogue based there really is no need for a narrative device. I do think that a conversation between Edmund, Jill and Eustace would be the least painful/awkward/intrusive way to connect HHB to the other stories in the "Tri-Star trilogy"(?), but it would serve no other purpose. A narrative that does not provide that connection would be a waste of screen time.