1. It's too bad we won't see the old design of Cair Paravel. I liked it...

2.I wish we could see Ben Barnes' Caspian and Will Poulter's Eustace reunited at the end. Both actors did a good job with their roles and I would have liked to see them perform that scene. I don't think it will be that emotionally resonant to see the reunion between the Mark Gordon Eustace and Caspian since their relationship was totally offscreen. (We'll barely have the Mark Gordon Caspian at all.)

1. I'm sorry to have to disagree with you on both counts as I have quoted. Yes the Old Cair Paravel in LWW was lovely to see. But by PC that Cair Paravel had been in ruins for some time, possibly having been demolished by the invading Telmarines. Whose acquaintance with cannon was well demonstrated in the Disney/Walden film leaving little that did not need repair or replacement, including the door to the treasure chamber. We saw the castle that Miraz inhabited, and we were still in nearby Beruna, the market town, when the film ended.

Silver Chair won't be a problem, really, since all we see is a frontage onto a wharf from which a ship is departing. All we see inside the castle is Jill's bedroom and a banqueting hall - please note that there are several such "banqueting halls" that could be borrowed for the occasion in UK, including the refectory at Christchurch College at Oxford, borrowed for the HP movies. At the end of the story we end up at the wharf again, possibly at the same tree where Glimfeather met them, and we see very little of Cair Paravel. I agree that in his lifetime Caspian had plenty of time to repair or renovate the place, and no doubt did so after his return in VDT, especially as by then he had found his Queen and would like her to preside over such redecorating to her own liking. HHB, if it ever gets done, only shows Tashbaan and Anvard, whilst Cair Paravel, as in the other two undone films is only a mentioned and never seen location.

2. There shouldn't be too much of a problem with Eustace, though I, too, will miss Will Poulter. But then the Mark Gordon Eustace will also reflect a whole different sort of Eustace from the Eustace before he was undragonned. And in the Walden VDT he seems to have spent quite a considerable time in that film as a dragon. Caspian, I also agree, is a different matter. As you admit, Caspian only appears briefly at a distance, in the beginning and dying and resurrected at the end. All we know of him will have to be part of the dialogue from Eustace's POV, and it will be whoever plays Eustace who will make the reunion at the end believable as the old man he sees dying at Cair Paravel's wharf, gradually returns to what he was when he was on the Dawn Treader or earlier? And here we have a three-way problem. What about the age to which Caspian returns to? As a young man? A teenager as he was at the Second battle of Beruna? Or a cheeky wretch of the pre-adolescent who played BBC PC's Caspian, depicting the sort of reunion we saw in the BBC television version of SC? What I am saying is though I'd prefer Ben Barnes' version and feel sure he could carry it off, there are two other actors who could reasonably play the part, especially Samuel West who played Caspian in BBC's television VDT. Or would all of them be too old? However I may miss Will Poulter and especially Ben Barnes, it will be up to the new appointees to produce something believable,

Reliefs: No mention of the Green Mist. If the Walden Media series was still going on, I'd be afraid that when the LOTGK threw incense on the fire, it would be create a familiar green smoke that wafted around our heroes' heads as they became brainwashed. Maybe she would even say that she was the one responsible for the Mist in the last movie. I suppose this scene could still use something similar in the Mark Gordon movie. But it wouldn't look exactly like it did the Voyage of the Dawn Treader. And that's good because there it looked like a cartoon skunk farted.

I would hope the "green smoke" scene in SC would look different from the Green Mist of Walden's VDT. The powder, itself, does have to be green of course, according to the book. But hopefully what results from it doesn't have to be green at all. Chemistry and pyrotechnics can surely come up with something appropriate. I, too, would hate to see again in SC, either Tilda Swinton, or the green mist as portrayed in VDT. And yes, if Walden had agreed to film Silver Chair, very likely there would be something blaming LOTGK for the green mist and a picture of her great aunt, or whatever, Tilda Swinton's The White Witch, on the wall in the room where was the Silver Chair. Thank goodness that won't happen.

Samuel West would be too old to play the rejuvenated Caspian. (For all I know, he might be dead now. )

I think Caspian should be roughly the same age in Aslan's Country as he was when Eustace knew him.

Sam West isn't dead. He was born in 1966, which makes him 52. In the future, go to IMDb for all things Hollywood.

https://www.imdb.com/name/nm0922335/