I must say, the situation you are facing is among the toughest one can face.
Your priorities are clear...get a job and stable housing to provide a place for your kids when you have them.
If it turns out the OM is abusive, then you'll be able, at least, to have a place for them to go.
Talk to the leaders in your church about this. Maybe they can help, get the word out about you needing a job, etc. Remember, the US is not at socialized as the UK, and places like Texas are even more lassiez-faire than the rest of the US in their social policies. It's the private sector that does the social work. The church is BIG in Texas, and many social services happen through it.
In general, talking to your MIL will not help. She's not an ally. You don't need to make your "peace" with her. It'll only create more tension with your wife. It's wasted energy.
Don't worry about your wife's Thanksgiving plans. If there's some way you can invite your kids (and maybe your wife) somewhere for Thanksgiving, do so. Again, wasted energy.
This is a marathon. All of us have wasted energy on fruitless paths. The main task is: take care of yourself. If you don't have a job or a home, you can't care for the kids. So take care of yourself SO that you can take care of the kids. Second task is take care of the kids. The only time you should "go after" your wife or OM is for the kids' well-being. You are not in a position to do that now. If social services takes the kids from your wife, they will be put with your MIL or foster parents unless you have a home. And child abuse is rampant in foster care.
If you can save youself and start really living, MAYBE your wife will turn around.
I'm deeply moved and deeply sorry this is happening to you.
