Whew! You really don't like your mil. That's OK but since you have to live in her home I suggest that you find a way to let go of your anger and learn ways to be diplomatic when talking with her. It sounds like you not only used an accusing tone of voice with her but you actually accused her of cracking the bed. Nearly everyone would respond to that defensively. You are fortunate that your mil is long suffering and only offered excuses. If I were her I'd be angered and fight back. I might also ask you to find another place to live.
Always give your mil a way to save face, a way out. It's not so important that she admit wrong as to prevent further damage. You could have told her that the bed is cracked and talk about the 100 lb weight limit without accusing her of anything.
It's back to the saying, "would you rather be right or be happy?"
As to the bed, if it's wood, I suggest that you unscrew the rails, force glue and wooden tooth picks into the holes, wrap the leg with heavy duty tape and redrill the holes. Then put the rail back on. This will reinforce the joining.
My granddaughter had a metal framed toddler bed and I sat on the edge some of the time. She jumped up and down on it too. It did not break. But if the bed is plastic or wood I do see that it could be cracked by extra weight caused by sitting on it or by a child jumping on it or from age. The point of this paragraph is to suggest to you that you can find ways of thinking of things that does not increase your anger with your mil.
I am sympathetic to your condition. It is really difficult to live with a mil. My daughter and her boyfriend and I have tried to live together. It was nearly impossible even tho we got along in a basic sort of way. They made an appointment with Adult and Family Services and found a way to get state help. They moved into subsidized housing. trained for jobs, received food stamps and baby sitting stipends which allowed them to become independent. I don't know your situation but I strong suggest that you find a way to move. This amount of dissension and anger is damaging to all of you.
I went back and read a couple of your past questions. You can probably get help from your state to set up your own household. Because you aren't working you would be eligible for training assistance that gives you more benefits than if you're living on a limited income. I suggest that by setting out on your own your marriage might have a better chance of eventually succeeding.