Wow, that's quite a "case study"! This woman you're interested in has been through the wringer, and even caused some of it. Obviously, despite that she has a history of weakness, immorality, and poor choices, you want her. Just so you know what you're signing up for.
You don't want my assessment, though. You want to know what the Bible would say about marrying her. Let me try to help.
I'll lead off with saying that the teachings of the Bible about divorce apply to God's people. There is no notion in the Bible that God was giving rules for all humanity. In the OT, He was talking to His covenant people when He talked about divorce (Dt. 24.1-4). In the NT, when Jesus talked about divorce, He referenced the OT ("You have heard it said..."). When Paul talked about divorce, he was talking to the Church about how Christians should behave. So the fact that she was doing this occasional sleeping around, and married and divorced—...
Question: You say she's a Christian (your second sentence). How long has she been a Christian? If I'm reading right...
- She was a Christian and when she was younger had sex with an unbeliever (disobeying the Scriptures) and left him.
- She was a Christian and married someone (you didn't say whether he was a believer or not) and got divorced. (listed in your hesitation #2)
- She committed adultery during her divorce proceedings.
- She married an atheist (disobeying the Scriptures). So is the first husband or a second one?
- She committed adultery again during divorce proceedings (or is this a review of the same story?)
It may not be my place to ask, but part of being a Christian is living the life of a Christian (Matt. 7.16): People are only Christians, not because they say they believe in God, but when they live like Christ (Jn. 14.15). Are you sure your girl is a Christian? I'm sure she says she is, but...
I'm not in agreement with the idea that sex = marriage. What about rape or sexual exploitation of any kind? The Bible us no reason to assert that sex = marriage. The Deuteronomy 22 text you mentioned has to do with protecting the woman financially and her reputation in the community. It's not a statement about sex = marriage, but we can talk more about this if you wish.
My study of Scripture, since this is what you want to know, shows that divorce can be considered legitimate if the offending person has violated the marriage vows. These are grounds for divorce. The Jews had a specific set of understandings behind marriages, and the Bible goes along those lines. Married people owed each other faithfulness in love, and material support (food, clothing, shelter). Divorce was legitimate for adultery, failure to provide, physical and emotional neglect, abandonment, and abuse.
We have similar understandings in our culture: divorce is legitimate in cases of sexual unfaithfulness, failure to provide materially, physical and emotional neglect, abandonment, and abuse.
Marriage was always intended to a permanent bond, but God allows divorce because people are so screwed up (Mt. 19.8). Divorce is never to be casual, easy, cavalier, quick, or thoughtless.
Those are some initial thoughts. Only you two know the details of her previous marriages and her sexual adventures. Only she knows the real status of her heart before God. Only you two know why she really got divorced (was it once, or twice?).
The Bible assumes marriage after divorce. We can go deeper into those texts if you want. But God doesn't want us changing partners like we change clothes. It's disrespectful and abusive to people, and it doesn't mirror God's faithfulness. He doesn't want us hooking up in times of weakness, trading partners on the fly, or moving in and out of marriages. He wants sexual faithfulness and committed love.
> Should she marry the fellow she slept with according to Exodus?
Absolutely not. That's not what Exodus is teaching. That's just adding one disaster to another.
> Is she still able to be married to me?
You've started the conversation (thank you), and I've started a reply. But there's still more to talk about. I need you to respond to some of the things I've said.
> I am not sure if he was a believer or not.
I'm not sure she is either, but that's for you to talk to me about.
> I just want to do the right thing.
This is great news. It's the foundation of a good person.
> I want our marriage to be blessed by God
The best way to ensure that is for both of you to have excellent relationships with God. We can talk about that also.
> help give me some clarity
Let's talk more. I'll assume you're not getting married tonight. Let's take some time to talk it through so that you're on solid ground.
> I have lost sleep over this and have horrible anxiety over it :/
Get a good night's sleep. Breathe. Let's talk.