Mortal sin requires one exterior, and two interior factors. The one exterior factor is "grave matter" something that is in grave opposition to God's Law. The two interior factors are sufficient knowledge, and the consent of the will.
Note it's not "perfect" knowledge or "perfect" consent, which are conditions available perhaps to angels, but not to us. The ordinary control that a normal person has over his or her own thoughts and actions, is sufficient for imputing the guilt of a mortal sin.
I remember when "Amnesia" was a theme of a lot of TV dramas. The character would get a good knock on the head, and then get baffled about his real identity and --- this was key to the drama -- forget who he was married to! ("Whoa! Who's my wife? Could it be you, pretty lady?") This led to all kinds of steamy situations until he finally got his brains unscrambled and figured out what was going on.
This person would arguably lack the cognitive capacity and interior freedom for a mortal sin of adultery.
Anybody else? Look, if you're past your 18th birthday and you've got sufficient intellect/maturity to assume monthly payments on a car or a cell phone, your mind and will are intact. You are NOT off the hook.
That's how I see it, anyway. God is the judge of interior states, but objectively, if you've divorced and remarried outside the Church, you didn't do so "inadvertently". You know the Church says --- and Jesus says, five times in the Gospels --- that that ain't right.
At least, that's how I see it.
Welcome to the remnant. Glad to have you on board. ;)
The Messiah did not meet, nor did the Apostle John write about, the Samaritan woman by chance. This is one of the most beautiful passages in the New Testament, being modeled after the story of Jacob and Rachel at the well. It speaks of communion with the Messiah.
The common assumption is the woman was divorced, rather than widowed, five times, but we do not really know the details of her story.
We know that the Messiah only told her to call her husband, to whom she was not actually married (not a legitimate husband), perhaps to give them a blessing after they asked Him for water. We do not know that portion of their tale from the Apostle.
That being said, His commandments are right and just and He gave the one holy catholic apostolic church has the power to bind and loose. Of course I wish I knew more of what happened to her. There is the Eastern Orthodox tradition of Saint Photini but the scriptures are silent. He had to pass through Samaria. So he came to a town of Samaria called Sychar, near the plot of land that Jacob had given to his son Joseph. Jacob’s well was there. Jesus, tired from his journey, sat down there at the well. It was about noon. A woman of Samaria came to draw water. Jesus said to her, “Give me a drink.” His disciples had gone into the town to buy food. The Samaritan woman said to him, “How can you, a Jew, ask me, a Samaritan woman, for a drink?” (For Jews use nothing in common with Samaritans.) Jesus answered and said to her, “If you knew the gift of God and who is saying to you, ‘Give me a drink,’ you would have asked him and he would have given you living water.” [The woman] said to him, “Sir, you do not even have a bucket and the well is deep; where then can you get this living water? Are you greater than our father Jacob, who gave us this well and drank from it himself with his children and his flocks?” Jesus answered and said to her,“Everyone who drinks this water will be thirsty again; but whoever drinks the water I shall give will never thirst; the water I shall give will become in him a spring of water welling up to eternal life.” The woman said to him, “Sir, give me this water, so that I may not be thirsty or have to keep coming here to draw water.” Jesus said to her, “Go call your husband and come back.” The woman answered and said to him, “I do not have a husband.” Jesus answered her, “You are right in saying, ‘I do not have a husband.’ For you have had five husbands, and the one you have now is not your husband. What you have said is true.” The woman said to him, “Sir, I can see that you are a prophet. Our ancestors worshiped on this mountain; but you people say that the place to worship is in Jerusalem.” Jesus said to her, “Believe me, woman, the hour is coming when you will worship the Father neither on this mountain nor in Jerusalem. You people worship what you do not understand; we worship what we understand, because salvation is from the Jews. But the hour is coming, and is now here, when true worshipers will worship the Father in Spirit and truth; and indeed the Father seeks such people to worship him. God is Spirit, and those who worship him must worship in Spirit and truth.” The woman said to him, “I know that the Messiah is coming, the one called the Anointed; when he comes, he will tell us everything.” Jesus said to her,“I am he, the one who is speaking with you.” At that moment his disciples returned, and were amazed that he was talking with a woman, but still no one said, “What are you looking for?” or “Why are you talking with her?” The woman left her water jar and went into the town and said to the people, “Come see a man who told me everything I have done. Could he possibly be the Messiah?” They went out of the town and came to him. Meanwhile, the disciples urged him, “Rabbi, eat.” But he said to them, “I have food to eat of which you do not know.” So the disciples said to one another, “Could someone have brought him something to eat?” Jesus said to them, “My food is to do the will of the one who sent me and to finish his work. Do you not say, ‘In four months the harvest will be here’? I tell you, look up and see the fields ripe for the harvest. The reaper is already receiving his payment and gathering crops for eternal life, so that the sower and reaper can rejoice together. For here the saying is verified that ‘One sows and another reaps.’ I sent you to reap what you have not worked for; others have done the work, and you are sharing the fruits of their work.” Many of the Samaritans of that town began to believe in him because of the word of the woman who testified, “He told me everything I have done.” When the Samaritans came to him, they invited him to stay with them; and he stayed there two days. Many more began to believe in him because of his word, and they said to the woman, “We no longer believe because of your word; for we have heard for ourselves, and we know that this is truly the savior of the world.”
John, Catholic chapter 4, Protestant verses four to forty two,
New American Bible, Revised Edition (NABRE)
boldness mine