I wonder whether MM would take again the role of Tom, who had a conscious breakdown in The Sleeper and with whom we could sympathize. The same could be said of KH or HS, who to us are the good guys.

If Tom, Zoe or Malcolm turn out to be the impersonator, would an actor accept to change his character from good to bad? It would be easier for Connie, Tessa, Juliet and Mace to come back. We expect them to be bad.

It might be the shock of Tom, Zoe or Malcolm turning from a good person into a bad person that Kudos has in store for the impersonator.

I don't see Erin as a CIA plant. She's got too much to lose and secondly she seems too much in awe of Harry and in need of his approval. I agree about some of the mistakes you've highlighted. Shows her inexperience to me, rather than treacherous intent. Her look at Collison to me is about acknowledging she killed a man (and I think she's not so experienced at that. Remember Zoe and Danny's reactions to killing the guy on the ferry? Not all the spies are Ros or Adam where death is concerned.

Callum is just cocky and inexperienced.

I think the blood group info could well have been Sacha, perhaps he has used it to determine his paternity due to recent suspicions/discovery of his mother's actions. One of the reasons he didn't shoot Harry at home perhaps? But Erin wouldn't have picked up on that, because she doesn't suspect that angle.

Not sure at all about what Coaver's agenda is. He could well be a smokescreen. Perhaps it's Ilya who contacted him?

But there has to be a CIA double cross somewhere, or it wouldn't be spooks

They'd love to have Hugh Laurie return. I remember someone saying Hugh Laurie was in London this spring, so I wonder if Jools Siviter might be making a guest appearance at some point? He was in MI 6 so would fit with the Russia storyline and apparently served there in the past.

which would include Tessa. Perhaps Tessa and Jools band together against Harry.

Jools, because Hugh Laurie was in London during the main part of Spooks filming. He was here to promote his new Blues album, but he would have had time to do a cameo. Plus Hugh Laurie is a massive name, even more so in America I believe.

Tariq in 10.2 or 10.3 could the old face be Malcolm? We really do not know whether the old face is a good person or a bad person.

I'm not an expert but my understanding is that blood group is not a very reliable indicator of parentage unless it's a very rare one and even then not conclusive proof.

Notice also there is a very brief shot of Erin looking very concerned when Harry challenges Callum about filing the message 'low priority'. I agree Callum is most likely naive and innocent, but did he file it on Erin's instructions?