Answers
1) What is the cause of his distant relationship with his wife?
Léon married her not out of love or affection, but because her father, an elderly lawyer, was having difficulty in gathering enough money to pay her dowry. As the man had taught Léon English as well as taken him up as apprentice, and to take over after him, Léon thought it would be the best way to repay the man, and so the two were married. Plans changed, and Léon moved to Paris with his new wife, which she despised, and although they had a son, the pair never saw eye-to-eye. Their personalities don't allow them to agree on anything. He never discussed his work with her, and when he left for England, she decided to stay in Paris with her friends and their son, who she has successfully managed to turn against him.

2) Is his relationship with Nell simply for pleasure or is there a romantic interest there?
I think at the beginning it is for pleasure- and because he can use Nell to get the information he needs out of certain English lords and nobles. He does, however, slowly begin to feel something other than that towards the boy.

3) How does the Ambassador feel about the rumours concerning his possibly corrupt way of politics?
Corrupt? Léon? HA! He would say that as a member of the Secret Police and support of the Committee for Public Safety, his politics is nothing but pure. It only seems corrupt to those who are corrupt themselves, and unable to view anything without seeing it as broken and tainted.

4) What is Léon's biggest regret in his life?
Possibly that he married. It has done him very little good, and he seems to have aged far to quickly in the last few years.

5) Does he prefer to live in England or France?
France. Of course, like the good honest Revolutionary he is!