I do not believe that Harry will go to Elena just after the investigation. It would be foolish and against his principles. If indeed he renews his relationship with Elena, I think it would be because he is trying to find out who is his impersonator. After all, she is married to the Russian Ambassador and it would be a liability for the British government if suddenly the Head of Section D had an affair with the wife of the Russian Ambassador. After all Harry is not Adam. I don't say it cannot happen, but it seems to me unlikely.
I suppose that Elena was trained to be a Soviet Union spy by the KGB which were merciless. Gulag or death awaited anybody who did not follow the party line. Harry knows the Soviet mentality and does not expect Elena to have changed her treacherous ways. I cannot believe that she was an innocent participant in the spying of her husband. Whatever one thinks of Ruth, she is not devious and has strong moral beliefs. I think that Harry has come to appreciate these qualities in Ruth.
As I said before, Kudos knows too well how to use the powerful chemistry between PF and NW. I doubt that another actress, however good, could break this relationship in one series. By that I mean we will see more scenes with Harry and Ruth than with Harry and Elena.