Wow, where to begin? I've been a fan of British sci-fi since H.G. Wells, Quatermass, classic Doctor Who, Moonbase 3, etc.
I was thinking if the transient beings were the same as the unstable trans-uranic beings, "not to be used where there is life". Perhaps the trans beings are Operators working for the evil (side of the) Entity of Time. That may explain their greater powers, and why the transients can only exist in the past, "where they belong" (the Triassic had dinosaurs but no humans) without external help. Sapphire and Steel are always trying to trap or destroy angry ghosts, beings from the beginning and end of Time, and Things from outside Time. In serial 2 Steel makes a highly dangerous deal with a more powerful adversary, angering Time in the process. In serial 4 our two Operators trap a creature inside a prism, a similar trap is used in serial 6 where three time zones become a trap, of which resentment (like in serial 2) is a powerful factor. Revenge of Time?
Which episode did the gorgeous Joanna Lumley/Sapphire stick out her tongue in a sensual way? I think it was serial 4 or 5.
You're half right. The Transients are not the Trans Uranics. They are Operators like Sapphire and Steel, but working for Time.
Although the elements in the series are only loosely named after elements in our periodic table, and some are not elements at all, we can assume that Trans Uranic means after uranium. If so they would be radioactive and kill any life they came into contact with. Not much use as operators then. Perhaps they were some kind of failed experiment by whoever or whatever created Sapphire and Steel. Operators that didn't work out. In Assignment One, Steel talks about the Trans Uranics as if they are some kind of failure, because they cannot be used on the job that he and Sapphire are doing.
The Transients are Operators for Time. They are not unstable but focused, organised and with a definite mission - to help Time break out wherever possible. Unlike Sapphire and Steel, Transients can travel through time because they can move up and down the time corridor, but they can only break out of the corridor and into our reality in the past.
In the documentary "Counting out Time", PJ Hammond specifically says that Sapphire and Steel cannot travel in time. They always work in the present. Therefore anything that can time travel will have an advantage over them. From this we can also conclude that their assignments happen in order.
Normally the Transients would be unable to cause any trouble as they work in the past and should never meet, but they worked out how to trap Sapphire and Steel by using people from the past as a lure, then the time box to imprison the present moment, and therefore S&S with it.
Originally Silver was going to be trapped in there as well and he would get them all out by doing, as PJ Hammond put it, "something with the cutlery drawer". It was changed to just S&S being trapped forever in the time box when the show wasn't going to get another season.
Certainly you can think of the trap as Time's revenge. It can't kill them so it traps them in a moment of the present, forever. Your resentment theory fits in very well there.
Inasmuch as anything is explained in this series, that's how I understand it. Hope it helps.
As for Joanna? I dunno, she's just gorgeous anyway, all the way through it. Love that blue dress.
In the commentary on the DVD he said it was changed because it was Sapphire and Steel's show not Silver's. He didn't say the knew it was the end. Like Blakes 7 ending it was supposed to be a cliff hanger to be resolved in the next series but no next series ever came.
Had there been a second series they would have been rescued by Silver. In the documentary (note, not commentary) he specifically says this. He said he closed the story and left them trapped, and wrote Silver out of it instead of having him stay and get them out.
The documentary probably came out after the DVDs with the commentary, and on the commentary people often only give half the story because they don't get time to fully explain things, and in the heat of the moment often misquote events. It isn't the first time it has happened. Somebody did raise the question of Silver becoming as important as S&S on their missions but it wasn't why they were left in the trap. They were left there because there would not be another series.
So he wrote an ending instead of a cliffhanger. Nothing to resolve, no rescue happening, just the end of their story.
Until I see his documentary I have to go with what was said in the commentary. It makes no sense that this documentary you talk about would contract the commentary. He had plenty of time to say they knew it was the end in the commentary. He could have said that instead of leaving Silver out because it wasn't his show.
And actually that ending is a cliffhanger, because it leaves the heroes in jeopardy that's never resolved. If Silver had rescued them then it would not havee been a cliff hanger.