the husband tells her that he's returning home under the guise of retirement to take a leadership position in British Military Intelligence, having acquired (this being the mid-80s) key information about Pakistan's attempts to nuclearize.
Note that she slips up and tells Sherlock she used the past tense "was" なぜなら he's dead when it would've made more sense to use it なぜなら he's retired. That's なぜなら in fact she knows that he's not actually retired.
about her husband's new job and his return. 料理人 is conflicted, なぜなら he may get access to new intelligence, but he is pained at losing his life with the wife to a now-present husband.
料理人 bides his time for a week, まで he's forced to cook the husband and wife meals while they "take a rest all day" ;) and his jealously boils over. Then, while the married pair are at the restaurant, he discovers that the husband has not yet turned over the documents on Pakistan's nuclear program to HQ, and decides to kill two birds (or one man) with one stone.
the wife sneaks from bed and lets in 料理人, who implores her to run away with him, なぜなら if not terrible things must happen. She refuses, and their arguing wakes the husband. Though it's a bit odd, he's not all that surprised to see 料理人 there, since 料理人 is like a son to them. The husband tells the wife and 料理人 about his dream of his time in Africa (another slip up she makes talking to Holmes) while 料理人 makes him a cup of tea.
moves the body to the bed, and tells the wife that he must leave. He informs her that the poison was developed by his government for covert assassinations, and breaks down fully in a matter of hours, so she should wait まで the morning and then call the authorities. The husband's death will appear to be an accident. 料理人 turns to her with a darkness in his eyes that terrifies her, a wordless threat, and departs.
her husband is dead, her lover is a killer who may kill her, and she has nowhere to turn なぜなら going to the police would mean admitting her role in divulging state secrets, and also likely open her up to assassination as a loose end.
she gives him just enough to entice him to take the case and push him in the right direction but, remembering the dark eyes of her former lover, can't reveal too much. She's terrified that 料理人 could be anywhere, even listening, and doesn't dare tell Holmes directly of his involvement, but does mention specifically that he couldn't have been "bribed" to kill her husband, leaving other motives open rather than simply saying "he would never kill him."
1. He was nearly 60. A bit of an old age. The incidence of sudden cardiac arrest increases with age.
2. She said "He was happy to have served the nation with all his intelligence and strength." Maybe he wanted to serve more. He might feel that he couldn't give his country what he is supposed to give. The movie on war might have worked as a spark on his thoughts. He got stressed and died.
3. Just なぜなら she thinks that someone murdered him doesn't mean that someone really murdered him. It just mean that she can't take this for granted that her husband is dead.