You asked for our thoughts, so this is my thought:
You are going to drive yourself crazy trying to figure out what goes on in her head.
You can't reason with unreasonable people, and there is no one more unreasonable than an addict ( that includes codependents not in recovery)
You can't possibly figure out why an addict thinks what they think, or does what they do ,unless you have suffered with addiction. I feel confident that a sober addict would give you the same advice. Stop trying to figure HER out. Focus on getting your own head and heart straight. I know you feel like you can't do that with out first knowing why she did the things she did... but yes you can. As long as you are focusing on her "whys" you aren't focusing on getting yourself better. I know this because I was stuck spinning my own wheels in the sand for a looooong time.
There is no reason you HAVE to engage with her when she comes crawling back. That's a choice you are making before it has even happened, and I don't think it's healthy for you to be waiting for it. It's like knowing you are going to get slapped in the face eventually so you are constantly on alert wondering when it will happen and how you will respond to it. Exhausting yes?
My wish for you is that you work hard at severing the ties that bind you to this toxic person, and that you soon come to the place of clarity you need to move forward away from the chaos.
Hugs