Some questions for you:
1. How did Hutch get into Christian's house?
2. If Hutch was inside Christian's house here, where did he go when they came inside?
I don't think we know the architecture of Boss's house very well. I don't think it is necessary. Wife sees the image and
is mortified. She believes it is real. It doesn't really matter what we think. But as with other times, she is willing to deal with the aftermath, if there be some, later.
3. If Hutch was actually there watching them, why did Vivian just carry on fucking, when every other time there has been the possibility (that she is aware of) of Hutch seeing them fuck, she has been terrified?
Terrified maybe, but certainly not to the point of stopping. How could she imagine, for instance that husband has not heard what was going on in Boss's bedroom? Certainly she was not concerned enough to stop, was satisfied that he pretended to be asleep. Never mind that no other woman in the history of the world would have put herself in that situation. If the sex is so compelling, there are better ways to have it.
4. Why would Hutch have gone out for the evening expecting a dinner, and not wore a jacket, but then to chase his wife to Christian's house, he suddenly decides he needs to go grab one because "the evening is cool"?
Hutch wasn't there, it was Vivian imagining him, based on her knowledge that he saw her fucking Christian there before.
There are greater and more significant improbabilities in this game than Husband's jacket. Like Wife going into Boss's bedroom to fuck in Husband's presence.
Even if you allow for different cars, different frames of mind, different driving styles, etc., and say that Hutch would have paused for long enough to go into the house and grab the jacket he wore to Christian's last time, he would have had to arrive at Christian's house almost an hour's drive away between one and two minutes after Vivian - otherwise he would have arrived either before she went round the back, or after they came into the house to continue fucking. That's a tiny window of time for all of that to pull together in. There are longshots, and then there is this...
I don't think we can be so certain of those probabilities, in the face of the fact that Wife has no image in her mind of him actually watching her fuck through a window.
She 100% knows she's been discovered. Since I started writing this response though,
Luc77 has provided this confirmation to you. Christian told her that Hutch was watching them from the window during the dinner evening. He saw it on his security cameras. That's where the image in Vivian's imagination comes from.
Again, that image is not in Wife's mind for her to drawn on. In other words, it can't be a flashback. Yes, flashbacks happen, conjuring complete images from conversation, not so much.
I think what Wife knows about what Husband has seen depends on what she is willing to admit to herself. Sometimes admitting what a person actually knows can be too painful to admit. This feels real to me given how conflicted Wife is about the whole affair.
I think Luk's comment earlier about manipulation applies. Wife is not herself. Is not consistent with her whole life before of loving a husband and tending to their child, is not acting like any other woman in the history of the world. One surprise along the way that SC might have in store for us, is when Boss decides to turn off the manipulation and see if Wife really wants him. I think if it is was going to be Boss, Wife would not feel so conflicted, would not be trying to hold on the Husband, would not be going home to Husband at the end of the day after Boss tells her to "stay." If Boss really lived in her heart, she would stay, and fuck him madly.