Helene *SPOILERS*

This is the first time Helene dies on screen in an adaptation. The 1956 and 1968 both mention her death (although the Russian version has her appear as a ghost in Andrei's long goodbye). The 1972 BBC Serial also has her death offscreen-and-mentioned, but it's the first to mention the cause. Of course, as commented before, she wasn't the big bad in those productions (just a cold cheating wife), so it wasn't really a cop-out to have her go offscreen.
The 2007 gives a different plotline for her. Like the new adaptation, the series developed her more as a major baddie who is out to get Natasha (probably because- in an added scene where Natasha tries to persuade Helene to stop the duel- she felt insulted by the girl's suggested belief in her infidelity). In this one, she and her father are in Moscow at invasion time and decide to appease Napoleon. She, of course, sleeps with an officer who gives her a disease that dooms her (her last scene is her breaking a mirror in realization).

The scene of public humiliation is really Davies providing Helene (which more wicked in this production than in the book) with a comeuppance worthy of Dickens at his most preachy.

certainly nothing anyone could accuse her of, without incontrovertible evidence. Helene could have and would have simply denied it as an outrageous calumny spread by her enemies; her husband was of course the father

I think Middleton caught Helene on the fly. She really is quite a good actress.

In Petersburg she had enjoyed the special protection of a grandee who occupied one of the highest posts in the Empire. In Vilna she had formed an intimacy with a young foreign prince. When she returned to Petersburg both the magnate and the prince were there, and both claimed their rights. Helene was faced by a new problem—how to preserve her intimacy with both without offending either.
What would have seemed difficult or even impossible to another woman did not cause the least embarrassment to Countess Bezukhova, who evidently deserved her reputation of being a very clever woman. Had she attempted concealment, or tried to extricate herself from her awkward position by cunning, she would have spoiled her case by acknowledging herself guilty. But Helene, like a really great man who can do whatever he pleases, at once assumed her own position to be correct, as she sincerely believed it to be, and that everyone else was to blame.
The first time the young foreigner allowed himself to reproach her, she lifted her beautiful head and, half turning to him, said firmly: "That's just like a man—selfish and cruel! I expected nothing else. A woman sacrifices herself for you, she suffers, and this is her reward! What right have you, monseigneur, to demand an account of my attachments and friendships? He is a man who has been more than a father to me!" The prince was about to say something, but Helene interrupted him.
"Well, yes," said she, "it may be that he has other sentiments for me than those of a father, but that is not a reason for me to shut my door on him. I am not a man, that I should repay kindness with ingratitude! Know, monseigneur, that in all that relates to my intimate feelings I render account only to God and to my conscience," she concluded, laying her hand on her beautiful, fully expanded bosom and looking up to heaven.
"But for heaven's sake listen to me!"
"Marry me, and I will be your slave!"
"But that's impossible."
"You won't deign to demean yourself by marrying me, you..." said Helene, beginning to cry.
The prince tried to comfort her, but Helene, as if quite distraught, said through her tears that there was nothing to prevent her marrying, that there were precedents (there were up to that time very few, but she mentioned Napoleon and some other exalted personages), that she had never been her husband's wife, and that she had been sacrificed.
"But the law, religion..." said the prince, already yielding.
"The law, religion... What have they been invented for if they can't arrange that?" said Helene.
The prince was surprised that so simple an idea had not occurred to him, and he applied for advice to the holy brethren of the Society of Jesus, with whom he was on intimate terms.

Did not watch the whole thing, but as to whether no one would know for sure depends on how long Pierre had been away.